drivers: serial: pl011: Fix ignored const qualifiers

When trying to build Zephyr with -Wignored-qualifiers this leads to a
compiler warning like:

zephyr/drivers/serial/uart_pl011_registers.h:40:1: error: type
qualifiers ignored on function return type [-Werror=ignored-qualifiers]

Since the return value is copied by the caller, making it const has no
effect and the compiler warns that this is being ignored. This enables
applications to build with additional compiler warnings turned on.

Signed-off-by: Benjamin Gwin <bgwin@google.com>