vexpress: Set ARMv7 reset values into SCTLR on boot

When running on ARMv8 with EL3 in Aarch64 the values of SCTLR_EL1 (which
OP-TEE then would use) are undefined. ARM-TF does clear and set all
undefined bit as expected. But for architectural correctness we clear
and set SCTLR as appropriate too.