ARM: KVM: convert GP registers from u32 to unsigned long

On 32bit ARM, unsigned long is guaranteed to be a 32bit quantity.
On 64bit ARM, it is a 64bit quantity.

In order to be able to share code between the two architectures,
convert the registers to be unsigned long, so the core code can
be oblivious of the change.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>