OMAP2+: hwmod: allow custom pre-shutdown functions

Some OMAP IP blocks, such as the watchdog timers, cannot be completely
shut down via the standard hwmod shutdown mechanism.  This patch
enables the hwmod data files to supply a pointer to a custom
pre-shutdown function via the struct omap_hwmod_class.pre_shutdown
function pointer.  If the struct omap_hwmod_class.pre_shutdown
function pointer is non-null, the function will be executed before the
existing hwmod shutdown code runs.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Benoît Cousson <b-cousson@ti.com>