clock: gic-timer: Add support for vdso_direct

This patch is a consequence of the introduction of vdso_direct on
mips architectures.

If the clock source is selected, vdso_direct is set to true.

Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>