arm64: zynqmp: Try to enable the first watchdog via aliases

arm64: zynqmp: Try to enable the first watchdog via aliases

Add support for enabling the first watchdog pointed via aliases.

DT fragment:
	aliases {
		...
		watchdog0 = &watchdog0;
		watchdog1 = &watchdog_lpd;
		...
	};

dm tree fragment for above configuration with patch applied:
ZynqMP> dm tree
 Class    index  Probed  Driver      Name
-----------------------------------------
...
 watchdog    0  [   ]   cdns_wdt    |   |-- watchdog@ff150000
 watchdog    1  [ + ]   cdns_wdt    |   `-- watchdog@fd4d0000
...

dm uclass fragment:
ZynqMP> dm uclass
...
uclass 75: watchdog
0   watchdog@ff150000 @ 7df02f40, seq -1, (req 1)
1 * watchdog@fd4d0000 @ 7df02ff0, seq 0, (req 0)
...

It is visible that index 1 is IP with seq 0 which means that FPD
watchdog (@fd4d0000) is in DT below LPD watchdog (@ff150000).

Till this patch the first watchdog found in DT was used and started
which is not enabling all possible configuration based on user request.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>