arm: mvebu: turris_omnia: enable A385 watchdog before disabling MCU watchdog

Commit aeb0ca64 ("arm: mvebu: turris_omnia: disable MCU watchdog in
SPL when booting over UART") disabled MCU watchdog when booting over
UART to ensure that watchdog does not reboot the board before UART
transfer finishes.

But if UART transfer fails for some reason, or if U-Boot binary crashes,
then board hangs forever as there is no watchdog running which could
reset it.

To fix this issue, enable A385 watchdog with very high timeout before
disabling MCU watchdog to ensure that even slow transfer can finish
successfully before watchdog timer expires and also to ensure that if
board hangs for some reason, watchdog will reset it.

Omnia's MCU watchdog has fixed 120 seconds timer and it cannot be
changed (without updating MCU firmware). A385 watchdog by default uses
25 MHz input clock and so the largest timeout value (2^32-1) can be
just 171 seconds. But A385 watchdog can be switched to use NBCLK (L2) as
input clock (on Turris Omnia it is 800 MHz clock) and in this case final
watchdog clock frequency is calculated as:

  freq = NBCLK / 2 / (2 ^ R)

So A385 watchdog on Turris Omnia can be configured to at most 1374
seconds (about 22 minutes). We set it to 10 minutes, which should be
enough even for bigger U-Boot binaries or slower UART transfers.

Both U-Boot and Linux kernel, when initializing A385 watchdog, switch
watchdog timer to 25 MHz input clock, so usage of NBCLK input clock in
U-Boot SPL does not cause any issues.

Fixes: aeb0ca64 ("arm: mvebu: turris_omnia: disable MCU watchdog in SPL when booting over UART")
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>