serial: sc16is7xx: improve regmap debugfs by using one regmap per port

With this current driver regmap implementation, it is hard to make sense
of the register addresses displayed using the regmap debugfs interface,
because they do not correspond to the actual register addresses documented
in the datasheet. For example, register 1 is displayed as registers 04 thru
07:

$ cat /sys/kernel/debug/regmap/spi0.0/registers
  04: 10 -> Port 0, register offset 1
  05: 10 -> Port 1, register offset 1
  06: 00 -> Port 2, register offset 1 -> invalid
  07: 00 -> port 3, register offset 1 -> invalid
  ...

The reason is that bits 0 and 1 of the register address correspond to the
channel (port) bits, so the register address itself starts at bit 2, and we
must 'mentally' shift each register address by 2 bits to get its real
address/offset.

Also, only channels 0 and 1 are supported by the chip, so channel mask
combinations of 10b and 11b are invalid, and the display of these
registers is useless.

This patch adds a separate regmap configuration for each port, similar to
what is done in the max310x driver, so that register addresses displayed
match the register addresses in the chip datasheet. Also, each port now has
its own debugfs entry.

Example with new regmap implementation:

$ cat /sys/kernel/debug/regmap/spi0.0-port0/registers
1: 10
2: 01
3: 00
...

$ cat /sys/kernel/debug/regmap/spi0.0-port1/registers
1: 10
2: 01
3: 00

As an added bonus, this also simplifies some operations (read/write/modify)
because it is no longer necessary to manually shift register addresses.
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://lore.kernel.org/r/20231030211447.974779-1-hugo@hugovil.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>