OMAP3 clock: add support for setting the divider for sys_clkout2 using clk_set_rate

Add clock framework support for changing the rate of sys_clkout2.

Signed-off-by: Laine Walker-Avina <lwalkera@ieee.org>
[paul@pwsan.com: added commit message, added .round_rate pointer]
Signed-off-by: Paul Walmsley <paul@pwsan.com>