irqchip/mips-gic: Add pr_fmt and reword pr_* messages

Several messages from the MIPS GIC driver include the text "GIC", but
the format is not standard. Add a pr_fmt of "irq-mips-gic: " and reword
the messages now that they will be prefixed with the driver name.

Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Reviewed-by: Paul Burton <paul.burton@mips.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>