omap2 clock: fix incorrect rate calculation for osc_ck, sys_ck

omap2_get_crystal_rate() calculates osc_ck and sys_ck rates
incorrectly.  osc_ck runs at the same rate as the external clock
source.  Also, sys_ck's rate derives from osc_ck's rate, divided (not
multiplied) by PRCM_CLKSRC_CTRL:SYSCLKDIV.

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