spi: sh-msiof: Match renesas,rx-fifo-size in DT bindings doc with driver

Commit fe78d0b7

 ("spi: sh-msiof: Fix FIFO size to 64 word from
256 word") changed the default RX FIFO size on R-Car Gen2 SoCs in the
driver code, but forgot to update the DT bindings documentation.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>