mtd: onenand: Fix unaligned access

Fix unaligned access in OneNAND core. The problem is that the ffchars[] array
is an array of "unsigned char", but in onenand_write_ops_nolock() can be passed
to the memcpy_16() function. The memcpy_16() function will treat the buffer as
an array of "unsigned short", thus triggering unaligned access if the compiler
decided ffchars[] to be not aligned.

I managed to trigger the problem with regular ELDK 5.4 GCC compiler.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Tom Rini <trini@ti.com>

drivers/mtd/onenand/onenand_base.c

@@ -91,7 +91,13 @@ static struct nand_ecclayout onenand_oob_32 = {
 	.oobfree	= { {2, 3}, {14, 2}, {18, 3}, {30, 2} }
 };
 
-static const unsigned char ffchars[] = {
+/*
+ * Warning! This array is used with the memcpy_16() function, thus
+ * it must be aligned to 2 bytes. GCC can make this array unaligned
+ * as the array is made of unsigned char, which memcpy16() doesn't
+ * like and will cause unaligned access.
+ */
+static const unsigned char __aligned(2) ffchars[] = {
 	0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 	0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,	/* 16 */
 	0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,