refactor(plat/allwinner): allow custom BL31 offset

Not all Allwinner SoCs have the same arrangement to SRAM A2.

Allow to specify a offset at which BL31 will stay in SRAM A2.

Change-Id: I574140ffd704a796fae0a5c2d0976e85c7fcbdf9
Signed-off-by: Icenowy Zheng <icenowy@sipeed.com>
3 files changed