xen/arm: vgic: Introduce helpers to extract/update/clear/set vGIC register ...

xen/arm: vgic: Introduce helpers to extract/update/clear/set vGIC register ...

and use them in the vGIC emulation.

The GIC registers may support different access sizes. Rather than open
coding the access for every registers, provide a set of helpers to access
them.

The caller will have to call vgic_regN_* where N is the size of the
emulated registers.

The new helpers supports any access size and expect the caller to
validate the access size supported by the emulated registers.

Finally, take the opportunity to fix the coding style in section we are
modifying.

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