This works around a limitation in our handling of the vexpress
config bus. For now just hardcode the clock parent to the 1MHz
clock, which is the default as emulated by QEMU and also the
setting the Linux kernel will configure later.

This fixes the vexpress clocksource running at a wrong rate
leading to bogus delays and sleep times.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>