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: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/Kconfig b/Kconfig
index 797038b..f7e3c33 100644
--- a/Kconfig
+++ b/Kconfig
@@ -305,6 +305,7 @@
 
 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