board/bsc9131rdb: Update default boot environment settings

BSC9131RDB has 1GB DDR.
Out of this, only 880MB is passed on to Linux via bootm_size.
Remaining
-16MB is reserved for PowerPC-DSP shared control area
-128MB is reserved for DSP private area.

Also 256MB, out of this 880MB is required for data communication between
PowerPC and DSP core.
For this bootargs are modified to pass parameter to create 1 hugetlb
page of 256MB via default_hugepagesz, hugepagesz and hugepages

Signed-off-by: Priyanka Jain <Priyanka.Jain@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>