sparc: merge LEON3 linker scripts

The linker scripts of LEON3

 - board/gaisler/grsim/u-boot.lds
 - board/gaisler/gr_cpci_ax2000/u-boot.lds
 - board/gaisler/gr_ep2s60/u-boot.lds
 - board/gaisler/gr_xc3s_1500/u-boot.lds

are the same (except cosmetic differences such as indentation).

This commit merges them into arch/sparc/cpu/leon3/u-boot.lds.

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