arm64, irqchip/gic-v3, ACPI: Move MADT GICC enabled check into a helper

ACPI, irqchip and the architecture code all inspect the MADT
enabled bit for a GICC entry in the MADT.

The addition of an 'online capable' bit means all these sites need updating.

Move the current checks behind a helper to make future updates easier.

Signed-off-by: James Morse <james.morse@arm.com>