arm: dts: rz-g2-beacon-u-boot: Fix QSPI Regression

The QSPI is accessed via the RPC-IF, but the compatible flags
previously used a different name.  This compatibel name was changed
which broke the ability to access the QSPI.  Fix this by removing
the custom naming reference.

Fixes: 68083b89 ("renesas: Fix RPC-IF compatible values")
Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Marek Vasut <marek.vasut+renesas@gmail.com>