The calculation for the Master clock divisor (MDIV) is different on the
SAM9 processors than on the AT91RM9200.

Orignal patch from Sascha Erlacher.

Also use the defined AT91_PMC_PRES instead of hard-coded bitmasks.
Signed-off-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>