Add support for DLVR (Digital Linear Voltage Regulator) attributes,
which can be used to control RFIM.

Here instead of "fivr" another directory "dlvr" is created with DLVR
attributes:

/sys/bus/pci/devices/0000:00:04.0/dlvr
├── dlvr_freq_mhz
├── dlvr_freq_select
├── dlvr_hardware_rev
├── dlvr_pll_busy
├── dlvr_rfim_enable
└── dlvr_spread_spectrum_pct
└── dlvr_control_mode
└── dlvr_control_lock

Attributes
dlvr_freq_mhz (RO):
Current DLVR PLL frequency in MHz.

dlvr_freq_select (RW):
Sets DLVR PLL clock frequency.

dlvr_hardware_rev (RO):
DLVR hardware revision.

dlvr_pll_busy (RO):
PLL can't accept frequency change when set.

dlvr_rfim_enable (RW):
0: Disable RF frequency hopping, 1: Enable RF frequency hopping.

dlvr_control_mode (RW):
Specifies how frequencies are spread. 0: Down spread, 1: Spread in Center.

dlvr_control_lock (RW):
1: future writes are ignored.

dlvr_spread_spectrum_pct (RW)
A write to this register updates the DLVR spread spectrum percent value.
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
[ rjw: Subject edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>