clk: at91: fix programmable clock for sama5d2

The prescaler formula of the programmable clock has changed for sama5d2.
Update the driver accordingly.

Fixes: a2038077

 ("clk: at91: add sama5d2 PMC driver")
Cc: <stable@vger.kernel.org> # v4.20+
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
[nicolas.ferre@microchip.com: adapt the prescaler range,
		fix clk_programmable_recalc_rate, split patch]
Signed-off-by: Matthias Wieloch <matthias.wieloch@few-bauer.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>