bcm2708: fix uart1 parameters

System clock is 250MHz, but the device uses a non-standard
oversampling rate (8 instead of 16) hence the clock rate
has to be multiplied by 16/8 = 2.  Currently the clock
rate seems to be divided by 2.

Also correct the .type and .flags.

Signed-off-by: Jakub Kicinski <kubakici@wp.pl>