spi: sh-msiof: Add support for R-Car H2 and M2

Add support for the MSIOF variant in the R-Car H2 (r8a7790) and M2
(r8a7791) SoCs.

Binding documentation:
  - Add future-proof "renesas,msiof-<soctype>" compatible values,
  - The default for "renesas,rx-fifo-size" is 256 on R-Car H2 and M2,
  - "renesas,tx-fifo-size" and "renesas,rx-fifo-size" are deprecated for
    soctype-specific bindings,
  - Add example bindings.

Implementation:
  - MSIOF on R-Car H2 and M2 requires the transmission of dummy data if
    data is being received only (cfr. "Set SICTR.TSCKE to 1" and "Write
    dummy transmission data to SITFDR" in paragraph "Transmit and Receive
    Procedures" of the Hardware User's Manual).
  - As RX depends on TX, MSIOF on R-Car H2 and M2 also lacks the RSCR
    register (Receive Clock Select Register), and some bits in the RMDR1
    (Receive Mode Register 1) and TMDR2 (Transmit Mode Register 2)
    registers.
  - Use the recently introduced SPI_MASTER_MUST_TX flag to enable support
    for dummy transmission in the SPI core, and to differentiate from other
    MSIOF implementations in code paths that need this.
  - New DT compatible values ("renesas,msiof-r8a7790" and
    "renesas,msiof-r8a7791") are added, as well as new platform device
    names ("spi_r8a7790_msiof" and "spi_r8a7791_msiof").
  - The default RX FIFO size is 256 words on R-Car H2 and M2.

This is loosely based on a set of patches from Takashi Yoshii
<takasi-y@ops.dti.ne.jp>.

Signed-off-by: Geert Uytterhoeven <geert+renesas@linux-m68k.org>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Mark Brown <broonie@linaro.org>