KVM: arm64: Add basic support for POR_EL2

KVM: arm64: Add basic support for POR_EL2

S1POE support implies support for POR_EL2, which  we provide by

- adding it to the vcpu_sysreg enum
- advertising it as mapped to its EL1 counterpart in get_el2_to_el1_mapping
- wiring it in the sys_reg_desc table with the correct visibility
- handling POR_EL1 in __vcpu_{read,write}_sys_reg_from_cpu()

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20241023145345.1613824-32-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>