feat: enable `-Wsign-compare`

This enables the `-Wsign-compare` warning everywhere except inside the
`ASSERT_EQ` macro, which will be fixed in later commits.

Change-Id: I485a0e909f23264c662702d996f792e9411ecd2c
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/src/arch/aarch64/hypervisor/psci_handler.c b/src/arch/aarch64/hypervisor/psci_handler.c
index b1e1843..3532998 100644
--- a/src/arch/aarch64/hypervisor/psci_handler.c
+++ b/src/arch/aarch64/hypervisor/psci_handler.c
@@ -183,7 +183,7 @@
 					(uintreg_t)&cpu_entry, (uintreg_t)c, 0,
 					0, 0, SMCCC_CALLER_HYPERVISOR);
 			*ret = smc_res.func;
-		} while (*ret == PSCI_ERROR_ALREADY_ON);
+		} while (*ret == (uintreg_t)PSCI_ERROR_ALREADY_ON);
 
 		if (*ret != PSCI_RETURN_SUCCESS) {
 			cpu_off(c);