When TF is compiled for aarch32 MAX_VIRT_ADDR_SPACE_SIZE is 2^32 in some cases,
which makes the test (size) <= MAX_VIRT_ADDR_SPACE_SIZE a tautology because
uintptr_t is a 32 bit value. The cast remove the warning for clang.

Change-Id: I1345f3400f8fbbe4ffd3caa990a90e7ba593dba5
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>