mx31pdk: Remove unneeded config

mx31pdk: Remove unneeded config

Currently there are two config options for building a U-boot binary for MX31PDK:

make mx31pdk_config

or,

make mx31pdk_nand_config

mx31pdk_config was developed first when no NAND SPL support was available for
MX31 and it requires that the U-boot binary is loaded into RAM via JTAG and it
forces SKIP_LOWLEVEL_INIT.

mx31pdk_nand_config was added later and it allows booting from NAND Flash.

Leave just one config option called mx31pdk so that it produces a binary that can boot from NAND Flash.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>