timer: vt8500: Convert vt8500 to use CLKSRC_OF

This patch converts arch-vt8500 to make use of CLKSRC_OF. Doing so
removes the need for include/linux/vt8500_timer.h as vt8500_timer_init
no longer needs to be visible outside vt8500_timer.c

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>