libmraa: riscv64 not supported

Addresses buildbot errors

> CMake Error at CMakeLists.txt:193 (message):
>  Only x86, arm, mips, PERIPHERALMAN and mock platforms currently supported

Signed-off-by: Hirokazu MORIKAWA <morikw2@gmail.com>
(cherry picked from commit 8b777d2ec6c6c1f51c5bf84bef9cffa62fb4f965)

libs/libmraa/Makefile

@@ -54,7 +54,7 @@ endef
 define Package/libmraa
   $(call Package/libmraa/Default)
   TITLE:=Eclipse MRAA lowlevel IO C/C++ library
-  DEPENDS:=+libstdcpp +libjson-c @!arc @!armeb @!powerpc
+  DEPENDS:=+libstdcpp +libjson-c @!arc @!armeb @!powerpc @!riscv64
 endef
 
 define Package/libmraa/description