OMAP2+: clock: allow per-SoC clock init code to prevent clockdomain calls from clock code

The OMAP2/3 clock code was written to notify the clockdomain code when
the first clock in a clockdomain is enabled and when the last enabled
clock in a clockdomain is disabled.  OMAP4 requires a different
approach: the hwmod code needs to signal the clockdomain code when to
force-enable and auto-idle a clockdomain during the IP block enable
process.  The current conjecture is that once that hwmod sequence is
implemented, it will no longer be necessary for the clock code to call
into the clockdomain code for "optional clocks" on OMAP4.

Add a static flag to the OMAP2+ clock code, clkdm_control, that by
default preserves the OMAP2/3 behavior.  Also add a function,
omap2_clk_disable_clkdm_control(), intended to be called from OMAP4
and beyond clock initcalls, that disables the old behavior.

Part of this patch was originally based on a patch by Rajendra Nayak
<rnayak@ti.com>.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Benoît Cousson <b-cousson@ti.com>
Cc: Rajendra Nayak <rnayak@ti.com>