These frequency calculations depend on the RCW format, which is not
dependent on any particular board. Switch to using ARCH symbols instead
of TARGET.
This whole function could probably use less ifdefs, but for now just do
a minimal conversion.
Fixes: 24cb6f2295 ("fsl-layerscape: Add fsl_esdhc peripheral clock support")
Signed-off-by: Sean Anderson <sean.anderson@seco.com>