ARM: GIC: private a standard get_irqnr_preamble assembler macro

Provide a standard get_irqnr_preamble assembler macro for platforms
to use, which retrieves the base address of the GIC CPU interface
from gic_cpu_base_addr.  Allow platforms to override this by defining
HAVE_GET_IRQNR_PREAMBLE.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>