earlycon: Initialize port->uartclk based on clock-frequency property

commit 814453adea7d081ad8917aa0f32d6a14165a3563 upstream.

On DT based platforms when current-speed property is present baudrate
is setup. Also port->uartclk is initialized to bogus BASE_BAUD * 16
value. Drivers like uartps/ns16550 contain logic when baudrate and
uartclk is used for baudrate calculation.

The patch is reading optional clock-frequency property to replace bogus
BASE_BAUD * 16 calculation to have proper baudrate calculation.

[-stable comment: commit 31cb9a85 ("earlycon: initialise baud field
 of earlycon device structure") has changed 8250_early.c behavior which
 now tries to setup UART speed. Ignoring clock-frequency results in
 wrong value of calculated divisor & malformed early console output.]

Fixes: 31cb9a85 ("earlycon: initialise baud field of earlycon device structure")
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
[rmilecki: add -stable comment and Fixes tag]
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>