tty: vt8500_serial: add support for UART in WM8880 chips

Newer WonderMedia chips introduced another flag in the UART line control
register, which controls whether RTS/CTS signalling should be handled in
the driver or by the hardware itself.

This patch ensures that the kernel can control RTS/CTS (including
disabling it altogether) by forcing this flag to software mode on affected
chips (only WM8880 so far).

Also remove the redundant copy of the binding doc, while we are here.

Signed-off-by: Alexey Charkov <alchark@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>