davinci: cpufreq: add support for keeping an additional clock constant

On OMAP-L138 SoC, some of the sysclks need not be at a fixed ratio
to CPU clock and can be kept at a relatively constant rate by
adjusting the PLLDIVn ratio even as cpufreq goes ahead and changes
the CPU clock.

This feature can be used to keep the EMIFA (PLL0 SYSCLK3) clock at a
constant rate so that the EMIF timings need not be re-programmed
whenever the CPU frequency changes.

This patch adds the required suppport to cpufreq driver.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>