sunxi: add config for SPL at 0x20000 on H6

On the new Allwinner H6 SoC, the SRAM A2 address (SPL load address) is
at 0x20000, which is different with any old Allwinner SoCs.

Add SPL position and size configuration for this.

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