dts: bindings: introducing new bindings for NXP's on-chip flash controllers

The binding 'nxp,iap-lpc' is used for different flash controllers,
preventing clarity. This commit introduces bindings for each version of
the flash controller, as they appear with NXP lpc device families:

nxp,iap-fmc11: for the flash controler used on the lpc11u6x family
nxp,iap-fmc54: for the flash controler used on the lpc54xx family
nxp,iap-fmc55: for the flash controler used on the lpc55xx family,
   except lpc553x
nxp,iap-fmc553: for the flash controler used on the lpc553x family

Signed-off-by: Yves Vandervennet <yves.vandervennet@nxp.com>