clk: iproc: Split off dig_filter

The PLL loop filter/gain can be located in a separate register on some
SoCs.  Split these off into a separate variable, so that an offset can
be added if necessary.  Also, make the necessary modifications to the
Cygnus and NSP drivers for this change.

Signed-off-by: Jon Mason <jonmason@broadcom.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>