efi: Register EFI rtc platform device only when available

Drop the separate driver that registers the EFI rtc on all EFI
systems that have runtime services available, and instead, move
the registration into the core EFI code, and make it conditional
on whether the actual time related services are available.

Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>