clk: at91: make use of syscon to share PMC registers in several drivers

The PMC block is providing several functionnalities:
 - system clk management
 - cpuidle
 - platform suspend

Replace the void __iomem *regs field by a regmap (retrieved using syscon)
so that we can later share the regmap across several drivers without
exporting a new specific API or a global void __iomem * variable.

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