KVM: arm64: Refactor HFGxTR configuration into separate helpers

A subsequent change will need to flip more trap bits in HFGWTR_EL2. Make
room for this by factoring out the programming of the HFGxTR registers
into helpers and using locals to build the set/clear masks.

Link: https://lore.kernel.org/r/20230609220104.1836988-3-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>