arm64: dts: r8a7795: salvator-x: Update memory node to 4 GiB map

This patch addes memory region:

  - After changes, the Salvator-X board has the following map:
    Bank0: 1GiB RAM : 0x000048000000 -> 0x0007fffffff
    Bank1: 1GiB RAM : 0x000500000000 -> 0x0053fffffff
    Bank2: 1GiB RAM : 0x000600000000 -> 0x0063fffffff
    Bank3: 1GiB RAM : 0x000700000000 -> 0x0073fffffff

  - Before changes, the old map looked like this:
    Bank0: 1GiB RAM : 0x000048000000 -> 0x0007fffffff

Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>