mfd: vexpress-sysreg: Use devres API variants

Use the managed devm_gpiochip_add_data() and devm_mfd_add_devices()
instead of their unmanaged counterparts. With this, no .remove() hook is
needed for driver unbind.

Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>