Introduce include/system-constants.h

Introduce include/system-constants.h

We have a number of CONFIG symbols today that are of the form:
SYM1 = CONST1 + CONST2
or other static math operations (shifts, etc).  The issue is that by
moving these to Kconfig we no longer have the ability to calculate these
values, so they become less flexible and useful.  It's also the case
that sometimes a platform will just define SYM1 directly or perform a
slightly different set of calculations.  We introduce this header now to
have a place to start to handle these cases.

Signed-off-by: Tom Rini <trini@konsulko.com>