MLK-11844-1 ARM: dts: imx6sx: reserve DDR space for M4 use case

MLK-11844-1 ARM: dts: imx6sx: reserve DDR space for M4 use case

FreeRTOS/M4 has DDR use case, M4 can only execute on DDR
space lower than 0xa0000000, Linux kernel will reserve 1M
(0x9ff00000 - 0x9fffffff) for it, below are the DDR usage:

0x80000000 - 0x9fefffff: Linux kernel;
0x9ff00000 - 0x9fffffff: FreeRTOS;
0xa0000000 - 0xbfefffff: Linux kernel;
0xbff00000 - 0xbfffffff: RPMSG shared memory;
0xc0000000 - 0xffffffff: Linux kernel if boards has 2GB DDR.

Also, remove 'reg' property which is unused.

Signed-off-by: Anson Huang <Anson.Huang@freescale.com>