ARM: at91/PMC: don't forget to write PMC_PCDR register to disable clocks

When introducing support for sama5d3, the write to PMC_PCDR register has
been accidentally removed.

Reported-by: Nathalie Cyrille <nathalie.cyrille@atmel.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: <stable@vger.kernel.org> # 3.10.x and later