sh: unify sh2/sh3/sh4 linker scripts

The linker scripts of sh2/sh3/sh4 are almost the same.
The difference among them is essentially only one line.

They can be consolidated into a single file, arch/sh/cpu/u-boot.lds
by re-writing the diffrent line as follows:

	KEEP(*/start.o	(.text))

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>