clk: at91: add PMC peripheral clocks

This patch adds new at91 peripheral clock implementation using common clk
framework.

Almost all peripherals provided by at91 SoCs need a clock to work properly.
This clock is enabled/disabled using PCER/PCDR resgisters.

Each peripheral is given an id (see atmel's datasheets) which is used to
define and reference peripheral clocks.

Some new SoCs (at91sam9x5 and sama5d3) provide a new register (PCR) where you
can configure the peripheral clock as a division of the master clock.
This will help reducing the peripherals power comsumption.

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>