board: rpi: move uart deactivation to board_init

When using OF_CONTROL, the disabled value of the mini UART platdata
gets reset after board_early_init_f. So move detection and disabling
to board_init and remove board_early_init_f.
This uses the first device using the mini uart driver, as this method
works reliably with different device trees or even no device tree at all.

Signed-off-by: Fabian Vogt <fvogt@suse.com>
Reviewed-by: Simon Glass <sjg@chromium.org>