at91: remove lowlevel_init.S

lowlevel_init.S is not used any more so remove it.
As consequence, we also don't have to generate u-boot.lds
but can use a static version as before.

This also fixes the out-of-tree build problem introduced
with commit f0a2c7b4 "at91: add support for the PM9263 board"

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Wolfgang Denk <wd@denx.de>