arm64: GICv3: introduce symbolic names for GICv3 ICC_SGI1R_EL1 fields

The gic_send_sgi() function used hardcoded bit shift values to
generate the ICC_SGI1R_EL1 register value.
Replace this with symbolic names to allow reusing them later.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>