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>

arch/sh/lib/zimageboot.c

@@ -45,6 +45,7 @@ int do_sh_zimageboot (cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 	bootargs = getenv("bootargs");
 
 	/* Clear zero page */
+	/* cppcheck-suppress nullPointer */
 	memset(param, 0, 0x1000);
 
 	/* Set commandline */