avr32: fix relocation address calculation

Commit 1865286466a5d0c7f2e3c37632da56556c838e9e (Introduce generic link
section.h symbol files) changed the __bss_end symbol type from char[] to
ulong. This led to wrong relocation parameters which ended up in a not working
u-boot. Unfortunately this is not clear to see cause due to RAM aliasing we
may get a 'half-working' u-boot then.

Fix this by dereferencing the __bss_end symbol where needed.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>