feat(zynqmp): resolve the misra 10.1 warnings

MISRA Violation: MISRA-C:2012 R.10.1
1) The expression of non-boolean essential type is being interpreted as a
boolean value for the operator.
2) The operand to the operator does not have an essentially unsigned type.

Signed-off-by: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@xilinx.com>
Change-Id: I97bbc056f4fee167742429e144144ba793bf77b3
diff --git a/plat/xilinx/zynqmp/plat_psci.c b/plat/xilinx/zynqmp/plat_psci.c
index 655a776..b7408b1 100644
--- a/plat/xilinx/zynqmp/plat_psci.c
+++ b/plat/xilinx/zynqmp/plat_psci.c
@@ -176,7 +176,7 @@
 {
 	VERBOSE("%s: power_state: 0x%x\n", __func__, power_state);
 
-	int32_t pstate = psci_get_pstate_type(power_state);
+	uint32_t pstate = psci_get_pstate_type(power_state);
 
 	assert(req_state);