Update imx platform to not rely on undefined overflow behaviour

This consists of ensuring that the left operand of each shift is
unsigned when the operation might overflow into the sign bit.

Change-Id: Ia0a10b4a30e63c0cbf1d0f8dfe5768e0a93ae1c7
Signed-off-by: Justin Chadwell <justin.chadwell@arm.com>

plat/imx/common/sci/imx8_mu.h

@@ -13,7 +13,7 @@
 #define MU_TR_COUNT1		4
 #define MU_RR_COUNT1		4
 
-#define MU_CR_GIEn_MASK1	(0xF << 28)
+#define MU_CR_GIEn_MASK1	(0xFu << 28)
 #define MU_CR_RIEn_MASK1	(0xF << 24)
 #define MU_CR_TIEn_MASK1	(0xF << 20)
 #define MU_CR_GIRn_MASK1	(0xF << 16)
@@ -23,7 +23,7 @@
 #define MU_SR_TE0_MASK1		(1 << 23)
 #define MU_SR_RF0_MASK1		(1 << 27)
 #define MU_CR_RIE0_MASK1	(1 << 27)
-#define MU_CR_GIE0_MASK1	(1 << 31)
+#define MU_CR_GIE0_MASK1	(1U << 31)
 
 #define MU_TR_COUNT			4
 #define MU_RR_COUNT			4