clk: pxa: unbreak lookup of CLK_POUT

Since switching to clk drivers, the CLK_POUT cannot be searched for by
clk_get() API and thus it returns with ENOENT.
Register it with the clk_lookup and thus unbreak the users of it.

Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>