arm64: Allow the arch timer to use the HYP timer

With the ARMv8.1 VHE, the kernel can run in HYP mode, and thus
use the HYP timer instead of the normal guest timer in a mostly
transparent way, except for the interrupt line.

This patch reworks the arch timer code to allow the selection of
the HYP PPI, possibly falling back to the guest timer if not
available.

Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>