ARM: KVM: fix HYP mapping limitations around zero

The current code for creating HYP mapping doesn't like to wrap
around zero, which prevents from mapping anything into the last
page of the virtual address space.

It doesn't take much effort to remove this limitation, making
the code more consistent with the rest of the kernel in the process.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@cs.columbia.edu>