davinci_nand chipselect/init cleanup

Update chipselect handling in davinci_nand.c so that it can
handle 2 GByte chips the same way Linux does:  as one device,
even though it has two halves with independent chip selects.
For such chips the "nand info" command reports:

  Device 0: 2x nand0, sector size 128 KiB

Switch to use the default chipselect function unless the board
really needs its own.  The logic for the Sonata board moves out
of the driver into board-specific code.  (Which doesn't affect
current build breakage if its NAND support is enabled...)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Scott Wood <scottwood@freescale.com>