mfd: vexpress-sysreg: set-up software nodes for gpio-mmio

mfd: vexpress-sysreg: set-up software nodes for gpio-mmio

Replace struct bgpio_pdata - that we plan to remove - with software
nodes containing properties encoding the same values thatr can now be
parsed by gpio-mmio.

Acked-by: Lee Jones <lee@kernel.org>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250701-gpio-mmio-pdata-v2-3-ebf34d273497@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>