mtd: spi-nor: allow to be built as module

There's no reason this can't be a module. Also, give SPI-NOR its own
submenu.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>

drivers/mtd/spi-nor/Kconfig

-config MTD_SPI_NOR
-	bool "SPI-NOR device support"
+menuconfig MTD_SPI_NOR
+	tristate "SPI-NOR device support"
 	depends on MTD
 	help
 	  This is the framework for the SPI NOR which can be used by the SPI