amd64_edac: Sanitize f10_get_base_addr_offset

This function maps the system address to the normalized DCT address.
Document what the code does for more clarity and wrap insane bitmasks in
a more understandable macro which generates them. Also, reduce number of
arguments passed to the function. Finally, rename this function to what
it actually does.

No functional change.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>