AArch64: Use negative values in error enumeration

With AArch64 the enumerations with values in the range
0x80000000 to 0x80000007 are being assigned to ssize_t
typed variables which are 64-bit rather than 32-bit, and
are being used in conditions where they are being checked
for being negative. This is not the case when ssize_t is
64-bit, so redefine this to INT32_MIN value.

Change-Id: I7a031a940a28658b3bf34bebac93dfb3ba397b05
Signed-off-by: Marcus Oakland <marcus.oakland@arm.com>
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>