clk: at91: make use of syscon/regmap internally

Use the regmap coming from syscon to access the registers instead of using
pmc_read/pmc_write. This allows to avoid passing the at91_pmc structure to
the child nodes of the PMC.

The final benefit is to have each clock register itself instead of having
to iterate over the children.

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