pl011: fix UARTRSR accesses corrupting the UARTCR value

Offset 4 is UARTRSR/UARTECR, not the UARTCR. The UARTCR would be
corrupted if the UARTRSR is ever written. Fix by implementing a correct
model of the UARTRSR/UARTECR register. Reads of this register simply
reflect the error bits in data register. Only breaks can be triggered in
QEMU. With the pl011_can_receive function, we effectively have flow
control between the host and the model. Framing and parity errors simply
don't make sense in the model and will never occur.

Signed-off-by: Rob Herring <rob.herring@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1395166721-15716-3-git-send-email-robherring2@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
