ath10k: add register access debugfs interface

Debugfs files reg_addr and reg_val are used for reading and writing to the
firmware (target) registers. reg_addr contains the address to be accessed,
which also needs to be set first, and reg_value is when used for reading and
writing the actual value in ASCII.

To read a value from the firmware register 0x100000:

# echo 0x100000 > reg_addr
# cat reg_value
0x00100000:0x000002d3

To write value 0x2400 to address 0x100000:

# echo 0x100000 > reg_addr
# echo  0x2400 > reg_value
#

Signed-off-by: Yanbo Li <yanbol@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>