powerpc: NAND: FSL UPM: document new bindings

This patch adds documentation for the new NAND FSL UPM bindings for:

 NAND: FSL-UPM: add multi chip support
 NAND: FSL-UPM: Add wait flags to support board/chip specific delays

It also documents the old binding for "chip-delay".

Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
Acked-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>