Update the OpenTitan interrupt layout to match the latest OpenTitan
bitstreams. This involves changing the Ibex PLIC memory layout and the
UART interrupts.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: e92b696f1809c9fa4410da2e9f23c414db5a6960.1617202791.git.alistair.francis@wdc.com