OMAP2+: hwmod: add ability to setup individual hwmods

Add omap_hwmod_setup_one(), which is intended for use early in boot to
selectively setup the hwmods needed for system clocksources and
clockevents, and any other hwmod that is needed in early boot.
omap_hwmod_setup_all() can then be called later in the boot process.
The point is to minimize the amount of code that needs to be run
early.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Benoît Cousson <b-cousson@ti.com>
Cc: Kevin Hilman <khilman@ti.com>
Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Tony Lindgren <tony@atomide.com>