cris: add arch/cris/include/asm/serial.h

Fix the following build errors:

  drivers/tty/serial/8250_early.c:160: error: 'BASE_BAUD' undeclared (first use in this function): 1 errors in 1 logs
  drivers/tty/serial/8250_early.c:37:24: error: asm/serial.h: No such file or directory: 1 errors in 1 logs

I am not sure if (1843200 / 16) is suitable for cris, but most other
arch's define it as this value.
Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Cc: Mikael Starvik <starvik@axis.com>
Cc: Jesper Nilsson <jesper.nilsson@axis.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/cris/include/asm/serial.h

+#ifndef _ASM_SERIAL_H
+#define _ASM_SERIAL_H
+
+/*
+ * This assumes you have a 1.8432 MHz clock for your UART.
+ */
+#define BASE_BAUD (1843200 / 16)
+
+#endif /* _ASM_SERIAL_H */