Fix compilation error when ENABLE_PIE=1

Fix compilation error when ENABLE_PIE=1

This patch fixes compilation errors when ENABLE_PIE=1.

<snip>
bl31/aarch64/bl31_entrypoint.S: Assembler messages:
bl31/aarch64/bl31_entrypoint.S:61: Error: invalid operand (*UND* section) for `~'
bl31/aarch64/bl31_entrypoint.S:61: Error: invalid immediate
Makefile:1079: recipe for target 'build/tegra/t194/debug/bl31/bl31_entrypoint.o' failed
<snip>

Verified by setting 'ENABLE_PIE=1' for Tegra platform builds.

Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>
Change-Id: Ifd184f89b86b4360fda86a6ce83fd8495f930bbc