tty: serial: Add CONSOLE_POLL support to SiFive UART

Add CONSOLE_POLL support for future KGDB porting.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>

Changes since v1:
1. Fix the compile error reported by kbuild test robot
Link: https://lore.kernel.org/r/1584492027-23236-1-git-send-email-vincent.chen@sifive.com



Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>