arm: dts: k3-am642-*: Mark the memory node with u-boot, dm-spl

Since commit dffdb1f8 ("board: ti: am64x: Use fdt functions
for ram and bank init") ddr_init() and dram_bank_init() have
switched to fdtdec for getting the memory configuration from
the am64xx dts files instead of using hardcoded values. This
requires an accessible memory node in SPL as we already have
in k3-am642-r5-evm.dts.

Make the memory node accessible in A53 SPL for both am642-sk
and am642-evm and in am642-sk R5 SPL.

Signed-off-by: Georgi Vlaev <g-vlaev@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>