riscv: drop unneeded -Wall addition

The top level Makefile adds -Wall globally:

  KBUILD_CFLAGS   := -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs \

For riscv, I see two "-Wall" added for compiling each object.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>