atmel_nand: don't require CONFIG_SYS_NAND_ENABLE_PIN

atmel_nand: don't require CONFIG_SYS_NAND_ENABLE_PIN

If NCE is hooked up to NCS3, we don't need to (and can't)
explicitly set the state of the NCE pin. Instead, the
controller asserts it automatically as part of a
command/data access. Only "CE don't care"-type NAND chips
can be used in this manner.

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Reinhard Meyer <u-boot@emk-elektronik.de>