power/reset: Make the vexpress driver optional on arm and arm64

The driver can be used on either arm or arm64 platforms, but
the latter doesn't have any platform-specific configuration
options, so it must be possible to manually enable the driver.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>