The SRAM in the Allwinner H6 SoC starts at 0x2000, with the last part
ending at 0x117fff (although with gaps in between).
So SUNXI_SRAM_SIZE should be 0xf8000, not 0x98000.

Fix this to map the arisc exception vector area, which we will need
shortly.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>