ENGR00317981: busfreq: fix the use of MT_MEMORY_NONCACHED

ENGR00317981: busfreq: fix the use of MT_MEMORY_NONCACHED

Since commit 2e2c9de207be (ARM: add permission annotations to MT_MEMORY*
mapping types), MT_MEMORY_NONCACHED is not available any more.  Thus, we
see following build error.

  CC      arch/arm/mach-imx/busfreq_ddr3.o
arch/arm/mach-imx/busfreq_ddr3.c: In function ‘init_mmdc_settings’:
arch/arm/mach-imx/busfreq_ddr3.c:464:7: error: ‘MT_MEMORY_NONCACHED’ undeclared (first use in this function)

Fix it by using the new enum MT_MEMORY_RWX_NONCACHED.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>