OMAP4: DPLL cascading: Introduce menuconfig option

New menuconfig option CONFIG_OMAP4_DPLL_CASCADING will be
used for DPLL cascading feature. All related code should
use it.

Change-Id: I221d1bf92419cc84fdd014b0d20f49f18691d3b1
Signed-off-by: Andrii Tseglytskyi <andrii.tseglytskyi@ti.com>