Until now, clock related code for old ralink SoCs was based in fixed clocks
using 'clk_register_fixed_rate' and 'clkdev_create' directly doing in code
and not using device tree at all for their definition. Including this driver
is an effort to be able to define proper clocks using device tree and also
cleaning all the clock and reset related code from 'arch/mips/ralink' dir.
This clock and reset driver covers all the ralink SoCs but MT7621 which is
the newest and provides gating and some differences that make it different
from its predecesors. It has its own driver since some time ago. The ralink
SoCs we are taking about are RT2880, RT3050, RT3052, RT3350, RT3352, RT3883,
RT5350, MT7620, MT7628 and MT7688. Mostly the code in this new driver has
been extracted from 'arch/mips/ralink' and cleanly put using kernel clock
driver APIs. The clock plans for this SoCs only talks about relation between
CPU frequency and BUS frequency. This relation is different depending on the
particular SoC. CPU clock is derived from XTAL frequencies.

Depending on the SoC we have the following frequencies:
* RT2880 SoC:
    - XTAL: 40 MHz.
    - CPU: 250, 266, 280 or 300 MHz.
    - BUS: CPU / 2 MHz.
* RT3050, RT3052, RT3350:
    - XTAL: 40 MHz.
    - CPU: 320 or 384 MHz.
    - BUS: CPU / 3 MHz.
* RT3352:
    - XTAL: 40 MHz.
    - CPU: 384 or 400 MHz.
    - BUS: CPU / 3 MHz.
    - PERIPH: 40 MHz.
* RT3383:
    - XTAL: 40 MHz.
    - CPU: 250, 384, 480 or 500 MHz.
    - BUS: Depends on RAM Type and CPU:
        + RAM DDR2: 125. ELSE 83 MHz.
        + RAM DDR2: 128. ELSE 96 MHz.
        + RAM DDR2: 160. ELSE 120 MHz.
        + RAM DDR2: 166. ELSE 125 MHz.
* RT5350:
    - XTAL: 40 MHz.
    - CPU: 300, 320 or 360 MHz.
    - BUS: CPU / 3, CPU / 4, CPU / 3 MHz.
    - PERIPH: 40 MHz.
* MT7628 and MT7688:
    - XTAL: 20 MHz or 40 MHz.
    - CPU: 575 or 580 MHz.
    - BUS: CPU / 3.
    - PCMI2S: 480 MHz.
    - PERIPH: 40 MHz.
* MT7620:
    - XTAL: 20 MHz or 40 MHz.
    - PLL: XTAL, 480, 600 MHz.
    - CPU: depends on PLL and some mult and dividers.
    - BUS: depends on PLL and some mult and dividers.
    - PERIPH: 40 or XTAL MHz.

MT7620 is a bit more complex deriving CPU clock from a PLL and an bunch of
register reads and predividers. To derive CPU and BUS frequencies in the
MT7620 SoC 'mt7620_calc_rate()' helper is used.

In the case XTAL can have different frequencies and we need a different
clock frequency for peripherals 'periph' clock in introduced.

The rest of the peripherals present in the SoC just follow their parent
frequencies.

With this information the clk driver will provide all the clock and reset
functionality from a set of hardcoded clocks allowing to define a nice
device tree without fixed clocks.
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>