arm64: dts: r8a7796: salvator-x: Update memory node to 4 GiB map

This patch updates memory region:

  - After changes, the new map of the Salvator-X board on R8A7796 SoC
    Bank0: 2GiB RAM : 0x000048000000 -> 0x000bfffffff
    Bank1: 2GiB RAM : 0x000600000000 -> 0x0067fffffff

  - Before changes, the old map looked like this:
    Bank0: 2GiB RAM : 0x000048000000 -> 0x000bfffffff

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