mtd: spi: renesas: Extract strobe delay setting code into separate function

Move strobe delay setting code into extra function and reflect the latest
setting in datasheet (R-Car Gen3 v2.20, R-Car V3U v0.50).
i.e. STRTIM[2:0] should be set to 110 (RCar M3-W) or 111 (Other products)

This is also a preparation for new R-Car Gen4 SoC which has 4-bits STRTIM

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> # Fix for RZ/A1