dt-bindings: pinctrl: aspeed-g6: add FWQSPI function/group

Add FWQSPI function/group to support QSPI mode on the dedicated
FWSPI interface.

Signed-off-by: Jae Hyun Yoo <quic_jaehyoo@quicinc.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220329173932.2588289-6-quic_jaehyoo@quicinc.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>