[MTD] maps/plat-ram: Avoid gcc 4.0 warning

The assignement of a "const char *" to a "char *" variable
is emitting a warning with gcc 4.0. We cannot change
mtd->name to "const char *" as we have dynamic assignements
of the name. So casting is the correct solution here

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>