ARM: vexpress: Cleanup cppcheck shifting warning

Fix below warning from cppcheck tool using BIT() macro:
"Shifting signed 32-bit value by 31 bits is undefined behaviour errors"

Signed-off-by: Phong Tran <tranmanphong@gmail.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>