ARM: vf610: Add ARM Global Timer clocksource option

Add the ARM Global Timer as clocksource/scheduler clock option and
use it as default scheduler clock. This leaves the PIT timer for
other users e.g. the secondary Cortex-M4 core. Also, the Global Timer
has double the precission (running at pheripheral clock compared to
IPG clock) and a 64-bit incrementing counter register. We still keep
the PIT timer as an secondary option in case the ARM Global Timer is
not available.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Bill Pringlemeir <bpringlemeir@nbsps.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>