at91: Add arch_preboot_os which disables PIT in a faster way

at91: Add arch_preboot_os which disables PIT in a faster way

When disabled the PIT runs until it reaches the CPIV value.
The Linux PIT driver stops the PIT and waits until it stopped. This can
take over 100ms. Simply stopping in u-boot isn't sufficient as the PIT
will still be running when Linux is waiting until it stopped.
So, we stop it in u-boot by setting the compare value to a value slightly
greater than the current running counter to make the PIT stopped in short
time.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>