adi_wdt: add get_timeout support and fix silent timeout overflow

[pamolloy]

[PATCH 0/6] watchdog: report effective timeout and add gettimeout command
adds a max_timeout_ms field to per-device uclass-plat data. Drivers set this
at probe time; wdt_start() then clamps the requested timeout to the hardware
maximum and prints both values when they differ, e.g.:

WDT:   Started watchdog@31008000 without servicing  (42s timeout, requested 60s)

adi_wdt was not updated as part of that series and needs a follow-up.

Problem

All ADI SC5xx defconfigs set CONFIG_WATCHDOG_TIMEOUT_MSECS=60000 (60 s).
WDOG_CNT is a 32-bit register. At typical sclk0 rates the 60 s request
overflows the register silently:

Platform	sclk0	Max timeout	At 60 s requested
SC58X	200 MHz	~42.9 s	truncated → ~14.9 s
Non-SC58X	125 MHz	~34.4 s	truncated → ~25.6 s
Non-SC58X	200 MHz	~21.5 s	truncated → ~17.1 s

The boot log always prints 60s timeout regardless, so the discrepancy is
invisible.

Work Required

1. Set max_timeout_ms in adi_wdt_probe()

Once the upstream uclass change has landed, compute the hardware maximum during
probe and store it in the per-device uclass-plat data so the uclass can clamp
and report accurately:

/* in adi_wdt_probe(), after clk_get_by_name() succeeds */
struct wdt_uclass_plat *uc_plat = dev_get_uclass_plat(dev);
u64 clk_rate = clk_get_rate(&priv->clock);

if (IS_ENABLED(CONFIG_SC58X))
    clk_rate /= 2;

if (clk_rate)
    uc_plat->max_timeout_ms = lldiv((u64)U32_MAX * 1000, clk_rate);

2. Clamp WDOG_CNT and use lldiv() in adi_wdt_start()

The existing calculation overflows to a truncated u32 without clamping.
Fix it to match what the uclass will advertise:

u64 clk_rate = clk_get_rate(&priv->clock);

if (IS_ENABLED(CONFIG_SC58X))
    clk_rate /= 2;

u64 cnt = lldiv(timeout_ms * clk_rate, 1000);
if (cnt > U32_MAX)
    cnt = U32_MAX;

iowrite32((u32)cnt, priv->wdt_base + WDOG_CNT);

Acceptance Criteria

• Boot log on an SC5xx board with a 60 s requested timeout shows the clamped
  effective timeout, e.g. (42s timeout, requested 60s).
• WDOG_CNT contains the correctly clamped value, not a silently wrapped one.

[pamolloy]

[PATCH v2 0/5] watchdog: report effective timeout