arm: dts: rz-g2-beacon-u-boot: Enable pinmux for QSPI

When booting from QSPI, the boot ROM appears to mux the QSPI
pins, but it's not guaranteed to be setup when booting from
eMMC.  Fix this by explicitly configuring the pinmux.

Signed-off-by: Adam Ford <aford173@gmail.com>