MX51: removed warnings for the mx51evk

The patch removes warnings at compile time and provides
some cleanup code:
- Removed comment on NAND (not yet supported) from lowlevel_init.S
- Removed NFMS bit definition from imx-regs.h
  The bit is only related to MX.25/35 and can lead to confusion
- Moved is_soc_rev() to soc specific code (removed from mx51evk.c)

Signed-off-by: Stefano Babic <sbabic@denx.de>