sh4: use single u-boot linker script for all boards

sh4: use single u-boot linker script for all boards

Three supported SH4/SH4A boards with the bootloader image stored on
SPI flash have own flavour of a linker script, in turn they are equal
among each other. The only difference is that the text from
lowlevel_init.o is placed right after start.o, which makes sense.

Note that .bss section is not marked as NOLOAD, because for about
10 years this is a default option of a GNU linker, either the
attribute is found or not the resulting image file is the same.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>