xen/arm: traps: Correctly interpret the content of the register HPFAR_EL2

xen/arm: traps: Correctly interpret the content of the register HPFAR_EL2

The register HPFAR_EL2 (resp. HPFAR on arm32) contains the bits [47:12]
(resp. [39:12]) of the faulting IPA. Unlike other registers that represent
an address, the upper bits of the IPA are stored in the register bits
[4:39] (resp. [4:21]).

However, Xen assumes that the register contains the faulting IPA correctly
offsetted. This will result to get a wrong IPA when the fault is happening
during a translation table walk. Note this is only affecting  memaccess.

Introduce a new helper to get the faulting IPA from HPFAR_EL2 and
replace direct read from the register by the helper.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>