clock34xx.c: fix spinlock reentry in omap3_noncore_dpll_enable()

omap3_noncore_dpll_enable() calls clk_get_rate(), which causes an attempt
to re-enter the clockfw_lock spinlock.  Fix by calling
omap2_get_dpll_rate() instead.

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