arm: pdu001: Fix early debugging UART

The changes from commit 0dba4586 ("arm: Init the debug UART")
prevent the early debug UART from being initialized correctly.
To fix this we not just configure the pin multiplexer but add setting up
early clocks.

Signed-off-by: Felix Brack <fb@ltec.ch>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>