ARM: OMAP2+: timer: use a proper interface to get hwmod data

arch/arm/mach-omap2/timer.c pokes around inside the hwmod data
structures.  Since the hwmod data structures are about to change, this
code will break.  This patch modifies the timer code to use
recently-added hwmod functions instead.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Tony Lindgren <tony@atomide.com>