clk: at91: fix at91sam9x5 peripheral clock number

nck() looks at the last id in an array and unfortunately,
at91sam9x35_periphck has a sentinel, hence the id is 0 and the calculated
number of peripheral clocks is 1 instead of a maximum of 31.

Fixes: 1eabdc2f

 ("clk: at91: add at91sam9x5 PMCs driver")
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: <stable@vger.kernel.org> # v4.20+
Signed-off-by: Stephen Boyd <sboyd@kernel.org>