nitrogen6x: Fix RAM size variable

Fix the following build error when buildig nitrogen6s1g:

nitrogen6x.c:89:17: error: 'CONFIG_DDR_MB' undeclared (first use in
this function)
nitrogen6x.c:89:17: note: each undeclared identifier is reported only
once for each function it appears in

Reported-by: Albert Aribaud <albert.u.boot@aribaud.net>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>