mx51evk: Update environment in order to allow booting a dt kernel

mx51evk: Update environment in order to allow booting a dt kernel

Update the environment as done in other imx boards to allow easy switching
between booting a non-dt kernel and a dt kernel.

Change CONFIG_LOADADDR to 0x92000000, so that we can have the:

- uImage at 0x92000000
- imx51-babbage.dtb at 0x91000000

,which are adequate locations in RAM to avoid overlapping.

Boot tested the following kernels:

- 2.6.35 from FSL (11.09 branch)
- 3.9-rc7 non-dt
- 3.9-rc7 dt

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>