staging/goldfish: Add MTD dependency to KCONFIG

If CONFIG_MTD is not set goldfish_nand fails to compile with the
following linker warnings:

drivers/built-in.o: In function `goldfish_nand_remove':
goldfish_nand.c:(.text+0x6e7d0e): undefined reference to
`mtd_device_unregister'
drivers/built-in.o: In function `goldfish_nand_erase':
goldfish_nand.c:(.text+0x6e8ba2): undefined reference to
`mtd_erase_callback'
drivers/built-in.o: In function `goldfish_nand_init_device':
goldfish_nand.c:(.text+0x6e8eba): undefined reference to
`mtd_device_parse_register'

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>