lib: serial: Cadence: Enable compatibility for cdns,uart-r1p8

The Cadence driver does not use the RX byte status feature and hence can
be advertised to be compatible with cdns,uart-r1p8 as well.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>