omap2 clock: return -EINVAL if no clock enable code; fix dpll_ck enable

If the clock framework can't figure out how to enable a clock, return
-EINVAL to indicate that the clock cannot be enabled.  Previously the
code returned 0, which indicated success.

Also fix the "clock.c: Enable for dpll_ck without enable code" boot
warning by marking dpll_ck as ALWAYS_ENABLED.  This is not technically
true, since the DPLL can be bypassed; but since we currently have no
software infrastructure to control it directly, ALWAYS_ENABLED is a
sufficient fiction for dpll_ck.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>