ARM: OMAP3: PM: cpuidle: optimize the clkdm idle latency in C1 state

For a power domain to idle all the clock domains in it must idle.
This patch implements an optimization of the cpuidle code by
denying and later allowing only the first registered clock domain
of a power domain, and so optimizes the latency of the low power code.

The functions _cpuidle_allow_idle and _cpuidle_deny_idle are
not used anymore and so are removed.

Signed-off-by: Jean Pihet <j-pihet@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>