powerpc/85xx: clean up and document the QE/FMAN microcode macros

Several macros are used to identify and locate the microcode binary image
that U-boot needs to upload to the QE or Fman.  Both the QE and the Fman
use the QE Firmware binary format to package their respective microcode data,
which is why the same macros are used for both.  A given SOC will only have
a QE or an Fman, so this is safe.

Unfortunately, the current macro definition and usage has inconsistencies.
For example, CONFIG_SYS_FMAN_FW_ADDR was used to define the address of Fman
firmware in NOR flash, but CONFIG_SYS_QE_FW_IN_NAND contains the address
of NAND.  There's no way to know by looking at a variable how it's supposed
to be used.

In the future, the code which uploads QE firmware and Fman firmware will
be merged.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>