add GPIO driver

Change-Id: I67a9abef078e7a62b34dfbd366b45c03892800cd
Signed-off-by: Po Xu <jg_poxu@mediatek.com>

1. Implement GIC600 driver support and init
2. Remove unused debug info

Signed-off-by: Greta Zhang <greta.zhang@mediatek.com>
Change-Id: I30c08c531e705debc029071e4e970048e261c386

- Add basic platform setup
- Add mt8192 documentation at docs/plat/
- Add generic CPU helper functions
- Add basic register address

Change-Id: Ife34622105404a8227441aab939e3c55c96374e9
Signed-off-by: Nina Wu <nina-cm.wu@mediatek.com>