RISC-V: use generic serial.h

Fixes this from allmodconfig:

drivers/tty/serial/earlycon.c:27:10: fatal error: asm/serial.h: No such file or directory
Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/include/asm/Kbuild

@@ -40,6 +40,7 @@ generic-y += resource.h
 generic-y += scatterlist.h
 generic-y += sections.h
 generic-y += sembuf.h
+generic-y += serial.h
 generic-y += setup.h
 generic-y += shmbuf.h
 generic-y += shmparam.h