arch: hexagon: include: asm: Kbuild: add generic "serial.h" in Kbuild

Add "serial.h" in Kbuild, or can not pass compiling with allmodconfig,
the related error:

    CC [M]  drivers/staging/speakup/speakup_acntpc.o
  In file included from drivers/staging/speakup/speakup_acntpc.c:33:0:
  drivers/staging/speakup/serialio.h:7:24: fatal error: asm/serial.h: No such file or directory

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Richard Kuo <rkuo@codeaurora.org>