mtd: lpddr: fix Kconfig dependency, for I/O accessors

Not all architectures implement a writel_relaxed() accessor. Hopefully
this will change eventually, but for now, this means lpddr2_nvm.c can't
compile on some architectures.

Let's add an ARM dependency for now, and leave a comment so maybe we can
change this in the future.
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Vincenzo Aliberti <vincenzo.aliberti@gmail.com>

drivers/mtd/lpddr/Kconfig

@@ -19,7 +19,8 @@ config MTD_QINFO_PROBE
 	    Flash products
 
 config MTD_LPDDR2_NVM
-	depends on MTD
+	# ARM dependency is only for writel_relaxed()
+	depends on MTD && ARM
 	tristate "Support for LPDDR2-NVM flash chips"
 	help
 	  This option enables support of PCM memories with a LPDDR2-NVM