plat/meson/gxl: BL31: remove BL2 dependency

Remove an assert() that assumes a specific value being passed from
BL2.  This value is dependent on BL2 version, so makes this assert()
not portable.

Suggested-by: Remi Pommarel <repk@triplefau.lt>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Change-Id: Ife3d934b2fa37fc1c66963dd4eb1afe2ca17d740