clocksource/drivers/mips-gic-timer: Add pr_fmt and reword pr_* messages

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

Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>