feat(hcx): initialize HCRX_EL2 to its default value

feat(hcx): initialize HCRX_EL2 to its default value

The value of register HCRX_EL2 is UNKNOWN out of reset. This can
affect the behavior in lower exception levels, such as traps to
EL2 due to a wrong configuration of the register upon reset.

This patch initializes the register at EL3 and disables all traps
related to it.

On the other hand, new fields have been introduced for HCRX_EL2,
which are now defined in this patch, so they can be used in
further development.

Signed-off-by: Juan Pablo Conde <juanpablo.conde@arm.com>
Change-Id: I0bf1e949aa0d3be9f227358ad088a1ecb96ce222