MTD: mxs_nand_spl: Redo the way nand_init initializes

MTD: mxs_nand_spl: Redo the way nand_init initializes

Currently the spl system calls nand_init which does nothing.
It isn't until an attempt to load from NAND that it gets initialized.
Subsequent attempts to load just skip the initialization  because
NAND is already initialized.

This moves the contents of mxs_nand_init to nand_init.  In the event
of an error, it clears the number of nand chips found.  Any
attempts to use nand will check if there are nand chips available
instead of actually doing the initialization at that time. If there
are none, it will return an error to the higher level calls.

Signed-off-by: Adam Ford <aford173@gmail.com>