ARM: at91: convert timer suspend/resume to clock_event_device

Move at91's timer suspend/resume functions from struct sys_timer
at91sam926x_timer into struct clock_event_device pit_clkevt. This
will allow the sys_timer suspend/resume fields to be removed, and
eventually lead to a complete removal of struct sys_timer.

Cc: Andrew Victor <linux@maxim.org.za>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>