OMAP4: DPLL cascading: Added dummy symbols to support external modules

The DPLL cascading is used in external modules, so if the DPLL cascading is
disabled, the external modules couldn't be added by reason the lack of
needed symbols.

Change-Id: I5d5e54e04dddbb20e60d2115c81f2c4362eccc5b
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>