spi: rspi: Document support for Renesas QSPI in Kconfig

As of commit 5ce0ba88

 ("spi: rcar: add
Renesas QSPI support on RSPI") the rspi driver handles Renesas QSPI, too,
but this was not reflected in the Kconfig help text.

Signed-off-by: Geert Uytterhoeven <geert+renesas@linux-m68k.org>
Signed-off-by: Mark Brown <broonie@linaro.org>