sh: share the correct version of start.S among all cpus

It is easy to note that SH2/SH3/SH4 start.S code is practically
the same with a minor difference for SH2 where a short data header is
present. To avoid unwanted code duplication and to automatically
convert SH2 and SH3 platforms to generic board support move fixed SH4
start.S into arch/sh/lib/start.S and share it among all platforms.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sh/Makefile b/arch/sh/Makefile
index ca55fac..14e0b66 100644
--- a/arch/sh/Makefile
+++ b/arch/sh/Makefile
@@ -2,7 +2,7 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-head-y := arch/sh/cpu/$(CPU)/start.o
+head-y := arch/sh/lib/start.o
 
 libs-y += arch/sh/cpu/$(CPU)/
 libs-y += arch/sh/lib/