sparc: merge LEON2 and LEON3 linker scripts

There is only one line diff between LEON2 and LEON3 linker scripts:

  -             arch/sparc/cpu/leon2/start.o (.text)
  +             arch/sparc/cpu/leon3/start.o (.text)

They can be written in the same way:

                */start.o (.text)

So, board/gaisler/grsim_leon2/u-boot.lds
and arch/sparc/cpu/leon3/u-boot.lds
can be merged into arch/sparc/cpu/u-boot.lds.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Daniel Hellstrom <daniel@gaisler.com>