spi: rspi: Add r8a77470 to the compatible list

Add r8a77470 to the list of examples with soctypes.
No driver change is needed as "renesas,qspi" will activate
the right code within the corresponding driver.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>