mtd: spear_smi: Enable compile testing

This driver does not contain any architecture specific code, enable
compile testing it.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200928155715.14370-1-miquel.raynal@bootlin.com

drivers/mtd/devices/Kconfig

@@ -91,7 +91,7 @@ config MTD_MCHP23K256
 
 config MTD_SPEAR_SMI
 	tristate "SPEAR MTD NOR Support through SMI controller"
-	depends on PLAT_SPEAR
+	depends on PLAT_SPEAR || COMPILE_TEST
 	default y
 	help
 	  This enable SNOR support on SPEAR platforms using SMI controller