cmd_fat.c: Note in fatread help about alignment requirements

When using the partial read feature of fatwrite the buffer we read into
can become unaligned not just due to initial location but the size of
our partial reads as well.  Make this clear in the help text.

Signed-off-by: Tom Rini <trini@ti.com>

common/cmd_fat.c

@@ -49,6 +49,9 @@ U_BOOT_CMD(
 	"      If 'pos' is omitted, 0 is used. 'pos' requires 'bytes'.\n"
 	"      'bytes' gives the size to load. If 'bytes' is 0 or omitted,\n"
 	"      the load stops on end of file.\n"
+	"      If either 'pos' or 'bytes' are not aligned to\n"
+	"      ARCH_DMA_MINALIGN then a misaligned buffer warning will\n"
+	"      be printed and performance will suffer for the load.\n"
 	"      All numeric parameters are assumed to be hex."
 );