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>