arm/gic: Get rid of READ/WRITE_SYSREG32

arm/gic: Get rid of READ/WRITE_SYSREG32

AArch64 registers are 64bit whereas AArch32 registers
are 32bit or 64bit. MSR/MRS are expecting 64bit values thus
we should get rid of helpers READ/WRITE_SYSREG32
in favour of using READ/WRITE_SYSREG.
We should also use register_t type when reading sysregs
which can correspond to uint64_t or uint32_t.
Even though many AArch64 registers have upper 32bit reserved
it does not mean that they can't be widen in the future.

Modify types of following members of struct gic_v3 to register_t:
-vmcr
-sre_el1
-apr0
-apr1

Add new macro GICC_IAR_INTID_MASK containing the mask
for INTID field of ICC_IAR0/1_EL1 register as only the first 23-bits
of IAR contains the interrupt number. The rest are RES0.
Therefore, take the opportunity to mask the bits [23:31] as
they should be used for an IRQ number (we don't know how the top bits
will be used).

Signed-off-by: Michal Orzel <michal.orzel@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>