sh: sh7757lcr: Add KEEP order to start.o section

The sh7757lcr has a local u-boot.lds because the sh7757lcr is only
supported the SPI booting.
This patch refers from the commit "sh: Add KEEP order to start.o section"
(commit ID: b52da2aed8c2c388661f369052a97d5b5c9ed00a).

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>

board/renesas/sh7757lcr/u-boot.lds

@@ -41,7 +41,7 @@ SECTIONS
 
 	.text :
 	{
-		arch/sh/cpu/sh4/start.o		(.text)
+		KEEP(arch/sh/cpu/sh4/start.o		(.text))
 		*(.spiboot1.text)
 		*(.spiboot2.text)
 		. = ALIGN(8192);