feat(mediatek): implement generic platform port

Implement mandatory platform port functions. Receive
boot arguments from bl2, populate bl33 and bl32 image
entry structs, call each MTK initcall levels
in these mandatory platform port functions.
After bl31_main exit and handover to 2nd boot loader,
mtk bl33 issues SMC and traps to TF-A to execute boot_to_kernel
and then handover to Linux kernel.

Signed-off-by: Leon Chen <leon.chen@mediatek.com>
Change-Id: I8d5a3511668fc749c4c71edf1ac700002cb5a9c8