spi: sh-msiof: Double maximum DMA transfer size using two groups

The maximum DMA transfer size is limited by the maximum values that can
be written to the word count fields (WDLENx) in the Transmit and Control
Data Registers (SITDR2/SIRDR2).  As all MSIOF variants support
transferring data of multiple (two or four) groups, the maximum size can
be doubled by using two groups instead of one, thus reducing setup
overhead for very large SPI transfers.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/bad522c76b8d225c195433977b22f95015cf2612.1747401908.git.geert+renesas@glider.be


Signed-off-by: Mark Brown <broonie@kernel.org>