clk: at91: pmc: move pmc structures to C file

pmc.c is now the only user of struct at91_pmc*, move their definition in
the C file.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>