sunxi: change SUNXI_HIGH_SRAM option to SUNXI_SRAM_ADDRESS

The new Allwinner H6 SoC has its SRAM A1 at neither 0x0 nor 0x10000, but
it's at 0x20000. Thus the SUNXI_HIGH_SRAM option needs to be refactored
to support this new configuration.

Change it to SUNXI_SRAM_ADDRESS, which holds the real address of SRAM
A1 in the memory map.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com>