Add mmio utility functions
Add mmio utility functions

- Add mmio 16 bits read/write functions.
- Add clear/set/clear-and-set utility functions.

Change-Id: I00fdbdf24af537424f8666b1cadaa5f77a2a46ed
Signed-off-by: Jimmy Huang <17b2f82a731dc28428cf577995da8b43f2f13d79@mediatek.com>

sbranden/arm-trusted-firmware,idl3r/arm-trusted-firmware,robixnai/arm-trusted-firmware,sandrine-bailleux/arm-trusted-firmware,idl3r/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,davwan01/arm-trusted-firmware,achingupta/arm-trusted-firmware,jcastillo-arm/arm-trusted-firmware,davwan01/arm-trusted-firmware,robixnai/arm-trusted-firmware,jcastillo-arm/arm-trusted-firmware,achingupta/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,sandrine-bailleux-arm/arm-trusted-firmware,sandrine-bailleux/arm-trusted-firmware,sbranden/arm-trusted-firmware,sandrine-bailleux-arm/arm-trusted-firmware