KVM: arm64: nv: Handle virtual EL2 registers in vcpu_read/write_sys_reg()

KVM internally uses accessor functions when reading or writing the
guest's system registers. This takes care of accessing either the stored
copy or using the "live" EL1 system registers when the host uses VHE.

With the introduction of virtual EL2 we add a bunch of EL2 system
registers, which now must also be taken care of:

- If the guest is running in vEL2, and we access an EL1 sysreg, we must
  revert to the stored version of that, and not use the CPU's copy.

- If the guest is running in vEL1, and we access an EL2 sysreg, we must
  also use the stored version, since the CPU carries the EL1 copy.

- Some EL2 system registers are supposed to affect the current execution
  of the system, so we need to put them into their respective EL1
  counterparts. For this we need to define a mapping between the two.

- Some EL2 system registers have a different format than their EL1
  counterpart, so we need to translate them before writing them to the
  CPU. This is done using an (optional) translate function in the map.

All of these cases are now wrapped into the existing accessor functions,
so KVM users wouldn't need to care whether they access EL2 or EL1
registers and also which state the guest is in.

Reviewed-by: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Co-developed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>