dt-bindings: serial: renesas,em-uart: Add RZ/V2M clock to access the registers

dt-bindings: serial: renesas,em-uart: Add RZ/V2M clock to access the registers

The RZ/V2M SoC has an additional clock to access the registers. The HW
manual says this clock should not be touched as it is used by the
"ISP Firmware".

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Link: https://lore.kernel.org/r/20220512072649.7879-1-phil.edworthy@renesas.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>