mwifiex: add support for FW memory read/write operations

This patch adds support for FW memory read/write operations via debugfs.
This is useful during debugging FW issues.

Examples:

For reading FW memory location:
echo r 0x01ac > /sys/kernel/debug/mwifiex/mlan0/memrw
cat /sys/kernel/debug/mwifiex/mlan0/memrw

For writing FW memory location:
echo w 0x01ac 0x55aa > /sys/kernel/debug/mwifiex/mlan0/memrw

Signed-off-by: Chin-ran Lo <crlo@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>