[ARM] 4604/2: AT91: Master clock divistor on SAM9

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>