ARM: gic-v3: Work around definition of gic_write_bpr1

A new accessor for gic_write_bpr1 is added to arch_gicv3.h in 4.9,
whilst the CP15 accessors are redifined in a separate branch.
This leads to a horrible clash, where the new accessor ends up with
a crap "asm volatile" definition.

Work around this by carrying our own definition of gic_write_bpr1,
creating a small conflict which will be obvious to resolve.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>