Export additional attributes:

ddr_data_rate (RO) : Show current DDR (Double Data Rate) data rate.
rfi_restriction (RW) : Show or set current state for RFI (Radio
			Frequency Interference) protection.

These attributes use mailbox commands to get/set information. Here
command codes are:
0x0007: Read RFI restriction
0x0107: Read DDR data rate
0x0008: Write RFI restriction
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210517061441.1921901-3-srinivas.pandruvada@linux.intel.com