OMAP2+: clockdomain: add clkdm_in_hwsup()

Add a new function, clkdm_in_hwsup(), that returns true if a clockdomain
is configured for hardware-supervised idle.  It does not actually read the
hardware; rather, it checks an internal flag in the struct clockdomain, which
is changed when the clockdomain is switched in and out of hardware-supervised
idle.  This should be safe, since all changes to the idle mode should
pass through the clockdomain code.

Based on a set of patches by Rajendra Nayak <rnayak@ti.com> which do
the same thing by checking the hardware bits.  This approach should be
faster and more compact.

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