Pl011: Added registers UART_RSR/UART_ECR

Pl011: Added registers UART_RSR/UART_ECR

UART_RSR shows errors with the transmission and UART_ECR can clear
those (according to PL011 Technical Reference Manual Revision r1p4).  As
these transmission errors never occur, they are implemented as RAZ/WI.

Both registers exist at the same offset 0x004. RSR is read-only, ECR is
write-only.

Signed-off-by: Maurice Becker <madnaurice@googlemail.com>
Change-Id: Ia9d13c90c65feccf3ecec36a782170755b1e1c02
Reviewed-on: https://gem5-review.googlesource.com/12686
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>