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: b52da2ae).

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