drivers/memory/Kconfig: Add CONFIG_OF dependency

drivers/memory/Kconfig: Add CONFIG_OF dependency

JZ4780_NEMC doesn't depend on OF, and if OF isn't enabled we get this
error:
drivers/memory/jz4780-nemc.c: In function ‘jz4780_nemc_num_banks’:
drivers/memory/jz4780-nemc.c:72:10: error: implicit declaration of
 function ‘of_read_number’; did you mean ‘down_read_nested’?
 [-Werror=implicit-function-declaration]
   bank = of_read_number(prop, 1);
          ^~~~~~~~~~~~~~
          down_read_nested

Make JZ4780_NEMC depend on OF.

Fixes: ab99e11062c1 ("memory: jz4780-nemc: Allow selection of this driver when COMPILE_TEST=y")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>