valgrind: Disable on Risc-V

There are no defined instruction sequences in include/valgrind.h for
Risc-V, so CONFIG_VALGRIND will do nothing on this arch (and possibly won't
compile?). Update Kconfig accordingly.
Signed-off-by: NSean Anderson <seanga2@gmail.com>
Reviewed-by: NHeinrich Schuchardt <heinrich.schuchardt@canonical.com>

Kconfig

@@ -305,6 +305,7 @@ config TPL_SYS_MALLOC_F_LEN
 
 config VALGRIND
 	bool "Inform valgrind about memory allocations"
+	depends on !RISCV
 	help
 	  Valgrind is an instrumentation framework for building dynamic analysis
 	  tools. In particular, it may be used to detect memory management bugs