plat-vexpress: build with -mfloat-abi=soft

OP-TEE on plat-vexpress does not support Neon. This patch prevents generating
Neon instructions when the cross-compiler is configured to use
'-mfloat-abi=hard' by default (such as arm-linux-gnueabihf-gcc).

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>