ARM: at91: introduce SAMA5 support

This patch introduces the SAMA5 support and a generic board file for SAMA5
devices. It also updates the PMC driver to manage clock division which is a
requirement since some peripherals can't work at the bus frequency on SAMA5.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>