Kconfig: Make all Kconfig encoding ascii

Some of Kconfigs are using utf-8 encoding because of used chars. Convert
all of them to ascii enconging. Based on discussion ASCII should be used in
general with the exception of names.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/mtd/spi/Kconfig b/drivers/mtd/spi/Kconfig
index d068b78..bedc4e9 100644
--- a/drivers/mtd/spi/Kconfig
+++ b/drivers/mtd/spi/Kconfig
@@ -246,7 +246,7 @@
 	  to erasing whole blocks (32/64 KiB).
 	  Changing a small part of the flash's contents is usually faster with
 	  small sectors. On the other hand erasing should be faster when using
-	  64 KiB block instead of 16 × 4 KiB sectors.
+	  64 KiB block instead of 16 x 4 KiB sectors.
 
 	  Please note that some tools/drivers/filesystems may not work with
 	  4096 B erase size (e.g. UBIFS requires 15 KiB as a minimum).