nitrogen: Use unsigned long to specify the total RAM size

When building for the nitrogen boards with 2GiB the following warning happens:

nitrogen6x.c:89:38: warning: integer overflow in expression [-Woverflow]

2GiB can not fit in 32-bits, so use ulong instead.

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