arm64: Define Cortex-A73 MIDR

As we're about to introduce a new workaround that is specific to
Cortex-A73, let's define the coresponding MIDR.

Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>