cppcheck cleanup: fix nullPointer errors

There are a number of places where U-Boot intentionally and legally
accesses physical address 0x0000, for example when installing
exception vectors on systems where these are located in low memory.

Add "cppcheck-suppress nullPointer" comments to silence cppcheck
where this is intentional and legal.

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/arch/sh/lib/zimageboot.c b/arch/sh/lib/zimageboot.c
index 86d3998..3fea5f5 100644
--- a/arch/sh/lib/zimageboot.c
+++ b/arch/sh/lib/zimageboot.c
@@ -45,6 +45,7 @@
 	bootargs = getenv("bootargs");
 
 	/* Clear zero page */
+	/* cppcheck-suppress nullPointer */
 	memset(param, 0, 0x1000);
 
 	/* Set commandline */