mtd: spi: renesas: Add R-Car Gen4 support

Support RPC SPI on R-Car Gen4 R8A779F0 S4 and R8A779G0 V4H SoCs.

Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Signed-off-by: Hai Pham <hai.pham.ud@renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
[Marek: Squash S4 and V4H patches, fix brackets around STRTIM2]