armv7: Use isb/dsb directly in start.S

Toolchains which do not directly support using "isb" and "dsb" directly
are no longer functionally supported in U-Boot. Furthermore, clang has
for a long time warned about using the alternate form that we were.
Update the code.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/cpu/armv7/start.S b/arch/arm/cpu/armv7/start.S
index 7d7aac0..69e281b 100644
--- a/arch/arm/cpu/armv7/start.S
+++ b/arch/arm/cpu/armv7/start.S
@@ -134,8 +134,8 @@
  */
 #if !CONFIG_IS_ENABLED(SYS_ICACHE_OFF)
 	mcr	p15, 0, r0, c7, c5, 0	@ invalidate icache
-	mcr     p15, 0, r0, c7, c10, 4	@ DSB
-	mcr     p15, 0, r0, c7, c5, 4	@ ISB
+	dsb
+	isb
 #endif
 
 	bx	lr
@@ -188,8 +188,8 @@
 	mcr	p15, 0, r0, c8, c7, 0	@ invalidate TLBs
 	mcr	p15, 0, r0, c7, c5, 0	@ invalidate icache
 	mcr	p15, 0, r0, c7, c5, 6	@ invalidate BP array
-	mcr     p15, 0, r0, c7, c10, 4	@ DSB
-	mcr     p15, 0, r0, c7, c5, 4	@ ISB
+	dsb
+	isb
 
 	/*
 	 * disable MMU stuff and caches