Tegra: common: fixup the bl31 code size to be copied at reset

If the CPU doesn't run from BL31_BASE, the firmware needs to be
copied from load address to BL31_BASE during cold boot. The size
should be the actual size of the code, which is indicated by the
__RELA_END__ linker variable.

This patch updates the copy routine to use this variable as a
result.

Signed-off-by: anzhou <anzhou@nvidia.com>
Change-Id: Ie3a48dd54cda1dc152204903d609da3117a0ced9