mfd: vexpress-sysreg: Support building as a module

Enable building the vexpress-sysreg driver as a module.

As deferred probe between the vexpress components works now, we don't
need to create struct devices early with of_platform_device_create().

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