OMAP3 SPL: Add identify_nand_chip function

A number of boards are populated with a PoP chip for both DDR and NAND
memory.  Other boards may simply use this as an easy way to identify
board revs.  So we provide a function that can be called early to reset
the NAND chip and return the result of NAND_CMD_READID.  All of this
code is put into spl_id_nand.c and controlled via CONFIG_SPL_OMAP3_ID_NAND.

Signed-off-by: Tom Rini <trini@ti.com>