arm64: KVM: Refactor kern_hyp_va to deal with multiple offsets

As we move towards a selectable HYP VA range, it is obvious that
we don't want to test a variable to find out if we need to use
the bottom VA range, the top VA range, or use the address as is
(for VHE).

Instead, we can expand our current helper to generate the right
mask or nop with code patching. We default to using the top VA
space, with alternatives to switch to the bottom one or to nop
out the instructions.

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