ARM: OMAP2+: clockdomain: add usecounting support to autoidle APIs

The previous implementation was racy in many locations, where the current
status of the clockdomain was read out, some operations were executed,
and the previous status info was used afterwards to decide next state
for the clockdomain. Instead, fix the implementation of the allow_idle /
deny_idle APIs to properly have usecounting support. This allows clean
handling internally within the clockdomain core, and simplifies the
usage also within hwmod.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>