iwlwifi: mvm: add debugfs for prph reg read/write

Allow reading/writing prph registers.

The address is set in the first argument
of the write operation. second argument
is optional and can be used for writing.

e.g.
echo '0xA01234 0x99' > /sys/kernel/debug/ieee80211/phy0/iwlwifi/iwlmvm/prph_reg
will write 0x99 into reg 0xA01234

cat /sys/kernel/debug/ieee80211/phy0/iwlwifi/iwlmvm/prph_reg
will show its current value (probably 0x99)

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>