ARM: OMAP2+: PRCM: remove omap2_cm_wait_idlest()

Now that all users of mach-omap2/omap2_cm_wait_idlest() have been removed,
delete the function and its supporting macros and prototypes.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Tested-by: Vaibhav Hiremath <hvaibhav@ti.com>