am335x_evm: Make NAND support modular

Give the user the ability to disable NAND support by defining
CONFIG_NO_NAND. This will allow custom hardware to easily support
this configuration.

Signed-off-by: Justin Waters <justin.waters@timesys.com>
[trini: Make apply on top of other series]
Signed-off-by: Tom Rini <trini@ti.com>