Fix mxc_hab documenation for DEK blob generation

Include/fsl_sec.h defines sec_in and sec_out, according to the
platform's endianess. Therefore, CONFIG_SYS_FSL_LE needs to be
declared in the configuration file of the target, in order to use
enable the DEK blob generation command. This requirement is not
explicit in the README.mxc_hab.
Signed-off-by: NUlises Cardenas <Ulises.Cardenas@freescale.com>

doc/README.mxc_hab

@@ -69,6 +69,7 @@ CONFIG_SECURE_BOOT
 CONFIG_SYS_FSL_SEC_COMPAT    4 /* HAB version */
 CONFIG_FSL_CAAM
 CONFIG_CMD_DEKBLOB
+CONFIG_SYS_FSL_LE
 
 Note: The encrypted boot feature is only supported by HABv4 or
 greater.