memory: jz4780-nemc: Do not build by default

Enabling the JZ4780_NEMC driver makes sense only for specific hardware -
the Ingenic SoC architecture.  It is not an essential driver for the SoC
support so do not enable it by default.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>