Enable -Wshift-overflow=2 to check for undefined shift behavior

The -Wshift-overflow=2 option enables checks for left bit shifts.
Specifically, the option will warn when the result of a shift will be
placed into a signed integer and overflow the sign bit there, which
results in undefined behavior.

To avoid the warnings from these checks, the left operand of a shift can
be made an unsigned integer by using the U() macro or appending the u
suffix.

Change-Id: I50c67bedab86a9fdb6c87cfdc3e784f01a22d560
Signed-off-by: Justin Chadwell <justin.chadwell@arm.com>