Config MCKR according to the datasheet sequence, or else it
will cause the MCKR configuration failed.

Remove timeout checking for clock configuration, if configure
the clock failed, let the system hang while not run in wrong
clock configuration.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Tested-by: Heiko Schocher <hs@denx.de>