hw/char/pl011: Support all interrupt lines

hw/char/pl011: Support all interrupt lines

The PL011 UART has six interrupt lines:
 * RX (receive data)
 * TX (transmit data)
 * RT (receive timeout)
 * MS (modem status)
 * E (errors)
 * combined (logical OR of all the above)

So far we have only emulated the combined interrupt line;
add support for the others, so that boards that wire them
up to different interrupt controller inputs can do so.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>