serial: a37xx: Switch to XTAL clock when booting Linux kernel

Unfortunately the UART driver in current Linux for Armada 3700 expects
UART's parent clock to be XTAL and calculats baudrate divisor according
to XTAL clock. Therefore we must switch back to XTAL clock before
booting kernel.

Implement .remove method for this driver with DM_FLAG_OS_PREPARE flag
set.

If current baudrate is unsuitable for XTAL clock then we do not change
anything. This can only happen if the user either configured unsupported
settings or knows what they are doing and has kernel patches which allow
usage of non-XTAL parent clock.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>