clk: at91: pmc: drop at91_pmc_base

at91_pmc_base is not used anymore, remove it along with at91_pmc_read and
at91_pmc_write.

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>