trivial: Fix misspelling of "firmware" in powerpc Makefile

Fix misspelling of "firmware" in powerpc Makefile

It's spelled "firmware".
Signed-off-by: Nick Andrew <nick@nick-andrew.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

arch/powerpc/boot/Makefile

@@ -208,7 +208,7 @@ image-$(CONFIG_DEFAULT_UIMAGE)		+= uImage
 #
 # Theses are default targets to build images which embed device tree blobs.
 # They are only required on boards which do not have FDT support in firmware.
-# Boards with newish u-boot firmare can use the uImage target above
+# Boards with newish u-boot firmware can use the uImage target above
 #
 
 # Board ports in arch/powerpc/platform/40x/Kconfig