MIPS: vdso: Utilize __pa() for gic_pfn

The GIC user offset is mapped into every process' virtual address and is
therefore part of the hot-path of arch_setup_additional_pages(). Utilize
__pa() such that we are more optimal even when CONFIG_DEBUG_VIRTUAL is
enabled, and while at it utilize PFN_DOWN() instead of open-coding the
right shift by PAGE_SHIFT.

Reported-by: Greg Ungerer <gerg@kernel.org>
Suggested-by: Serge Semin <fancer.lancer@gmail.com>
Fixes: dfad83cb ("MIPS: Add support for CONFIG_DEBUG_VIRTUAL")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Greg Ungerer <gerg@kernel.org>
Tested-by: Greg Ungerer <gerg@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>