xen: arm: handle accesses to CNTP_CVAL_EL0

xen: arm: handle accesses to CNTP_CVAL_EL0

All OSes we have run on top of Xen use CNTP_TVAL_EL0 but for
completeness we really should handle CVAL too.

In vtimer_emulate_cp64 pull the propagation of the 64-bit result into
two 32-bit registers out of the switch to avoid duplicating for every
register. We also need to initialise x now since previously the only
register implemented register was R/O.

While adding HSR_SYSREG_CNTP_CVAL_EL0 also move
HSR_SYSREG_CNTP_CTL_EL0 so it is sorted correctly.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>