spi: sh-msiof: Transfer size improvements and I2S

Merge series from Geert Uytterhoeven <geert+renesas@glider.be>:

This patch series (A) improves single transfer sizes in the MSIOF
driver, using two methods:
  - By increasing the assumed FIFO sizes, impacting both PIO and DMA
    transfers,
  - By using two groups, impacting DMA transfers,
and (B) lets the recently-introduced MSIOF I2S drive reuse the SPI
driver's register definitions.  All of this is covered with a thick
sauce of fixes for (harmless) bugs, cleanups, and refactorings.

Note that the driver uses the limitations as specified in the hardware
documentation.  For discovering the actual FIFO sizes, I wrote some
crude test code that can be found at [2].

This is based on spi/for-next and sound-asoc/for-next, and has been
tested on a variery of R-Car SoCs.

[1] https://lore.kernel.org/cover.1746180072.git.geert+renesas@glider.be
[2] https://git.kernel.org/pub/scm/linux/kernel/git/geert/renesas-drivers.git/log/?h=topic/msiof-fifo