watchdog: omap_wdt: improve watchdog reset path

Remove busy looping during watchdog reset.
Each polling of W_PEND_WTGR bit ("finish posted
write") after watchdog reset takes 120-140us
on BeagleBone Black board. Current U-Boot code
has watchdog resets in random places and often
there is situation when watchdog is reset
few times in a row in nested functions.
This adds extra delays and slows the whole system.

Instead of polling W_PEND_WTGR bit, we skip
watchdog reset if the bit is set. Anyway, watchdog
is in the middle of reset *right now*, so we can
just return.

This noticeably increases performance of the
system. Below are some measurements on BBB:
 - DFU upload over USB                 15% faster
 - fastboot image upload               3x times faster
 - USB ep0 transfers with 4k packets   20% faster

Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Tested-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Tested-by: Alex Kiernan <alex.kiernan@gmail.com>

drivers/watchdog/omap_wdt.c

@@ -53,16 +53,25 @@ void hw_watchdog_reset(void)
 {
 	struct wd_timer *wdt = (struct wd_timer *)WDT_BASE;
 
-	/* wait for posted write to complete */
-	while ((readl(&wdt->wdtwwps)) & WDT_WWPS_PEND_WTGR)
-		;
+	/*
+	 * Somebody just triggered watchdog reset and write to WTGR register
+	 * is in progress. It is resetting right now, no need to trigger it
+	 * again
+	 */
+	if ((readl(&wdt->wdtwwps)) & WDT_WWPS_PEND_WTGR)
+		return;
 
 	wdt_trgr_pattern = ~wdt_trgr_pattern;
 	writel(wdt_trgr_pattern, &wdt->wdtwtgr);
 
-	/* wait for posted write to complete */
-	while ((readl(&wdt->wdtwwps) & WDT_WWPS_PEND_WTGR))
-		;
+	/*
+	 * Don't wait for posted write to complete, i.e. don't check
+	 * WDT_WWPS_PEND_WTGR bit in WWPS register. There is no writes to
+	 * WTGR register outside of this func, and if entering it
+	 * we see WDT_WWPS_PEND_WTGR bit set, it means watchdog reset
+	 * was just triggered. This prevents us from wasting time in busy
+	 * polling of WDT_WWPS_PEND_WTGR bit.
+	 */
 }
 
 static int omap_wdt_set_timeout(unsigned int timeout)