Add support for PMOD_MTU3 macro to enable MTU3 node on RZ/G2UL SMARC
EVK.

The MTU3a PWM pins on PMOD0 are muxed with SPI1. Disable SPI1, when
PMOD_MTU3 macro is enabled.
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20230727081848.100834-6-biju.das.jz@bp.renesas.comSigned-off-by: Geert Uytterhoeven <geert+renesas@glider.be>