clk: at91: pmc: Support backup for programmable clocks

When an AT91 programmable clock is declared in the device tree, register
it into the Power Management Controller driver. On entering suspend mode,
the driver saves and restores the Programmable Clock registers to support
the backup mode for these clocks.

Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>