arm: socfpga: clock: Clean up pll_config.h

arm: socfpga: clock: Clean up pll_config.h

Extract the clock configuration horribleness caused by pll_config.h in
the following manner.

First of all, introduce a few new accessors which return values of
various clocks used in clock_manager.c and use them in clock_manager.c .
These accessors replace those few macros which came from pll_config.h
originally. Also introduce an accessor which returns the struct cm_config
default configuration for the clock manager used in SPL.

The accessors are implemented in a board-specific wrap_pll_config.c
file, whose sole purpose is to include the qts-generated pll_config.h
and provide only the necessary values to the clock manager.

The purpose of this design is to limit the scope of inclusion for the
pll_config.h , which thus far was included build-wide and poluted the
namespace. With this change, the inclusion is limited to just the new
wrap_pll_config.c file, which in turn provides three simple functions
for the clock_manager.c to use.

Signed-off-by: Marek Vasut <marex@denx.de>