irqchip: mips-gic: Provide VP ID accessor

Provide a gic_read_local_vp_id() function to read the VCNUM field of the
GICs local VP_IDENT register. This will be used by a further patch to
check that the value reported by the GIC matches up with the kernels
calculation.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12334/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>