spi: sh-msiof: Add core support for dual-group transfers

spi: sh-msiof: Add core support for dual-group transfers

All MSIOF variants support transferring data of multiple (2 or 4)
groups.  Add definitions for the register bits related to multiple
groups, and enhance sh_msiof_spi_set_mode_regs() to accept a second
group size.

For now the second group is unused.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/be75e20cfcd2a6c0d73ab09e0126f902911adc69.1747401908.git.geert+renesas@glider.be
Signed-off-by: Mark Brown <broonie@kernel.org>