irqchip: mips-gic: Remove gic_get_usm_range()

The MIPS VDSO code is no longer reliant upon the irqchip driver to
provide the address of the GIC's user-visible section via
gic_get_usm_range(). Remove the now-dead code.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17041/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>