OMAP2xxx: clock: fix low-frequency oscillator clock rate

The OMAP2420/2430 external 32-kHz low-frequency oscillator is a 32768
Hz oscillator, not a 32,000 Hz oscillator[1][2].  Fix this in the clock
tree.

Signed-off-by: Paul Walmsley <paul@pwsan.com>

1. OMAP2420/22 Multimedia Processor Data Manual, Version P [SWPS019P],
   section 5.1.4 "External 32-kHz CMOS Clock" (note that it refers to
   a "32.768-kHz" clock; this presumably should be "32.768-KHz")

2. OMAP2430 Multimedia Processor ES2.1 Data Manual, Version V [SWPS023V],
   section 5.1.4 "External 32-kHz CMOS Clock" (note that it refers to
   a "32.768-kHz" clock; this presumably should be "32.768-KHz")