ARM: at91/at91sam9x5: clock management for at91sam9x5 chip family

Several changes to PMC have to be managed for adding this support:
- alternate prescaler location for both MCKR and PCKR
- alternate CSS length for PCKR
- added cpu_is_at91sam9x5() to functional switches
- manage UTMI bias like sam9g45 chip family

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