OMAP4: DPLL: Add dpll api to control GATE_CTRL

On OMAP4, the dpll post divider outputs (MX outputs)
along with clockout_x2 output provide a way to allow/deny
hardware level autogating.
Allowing autoidle would mean that the hw would autogate
this clock when there is no dependency for it.
Denying idle would mean that this clock output will be
forced to stay enabled.

Add dpll api's to read/allow/deny idle control
for these dpll mx postdividers.

NOTE: The gatectrl bit set to 0 allows gatectrl,
and the bit set to 1 denies gatectrl.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
[paul@pwsan.com: moved OMAP4-specific DPLL control code to
 mach-omap2/dpll44xx.c; added some documentation for CLOCK_CLKOUTX2]
Signed-off-by: Paul Walmsley <paul@pwsan.com>