arm64: dts: renesas: salvator-common: Add RPC HyperFlash device node

Add the RPC HyperFlash device node along with its partitions to the
common Salvator-X(S) board DTS file.

Based on a patch in the BSP by Valentine Barshak.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/cfc6af8a4c42febcc405b7356c38448eec8e29b0.1648548339.git.geert+renesas@glider.be