watchdog: omap_wdt: Convert watchdog driver to use DT and DM

watchdog: omap_wdt: Convert watchdog driver to use DT and DM

This patch adds device tree and driver model watchdog support,
converts the legacy omap watchdog driver to driver model for
TI AM335x chipsets. The following compile warning is removed:

===================== WARNING ======================
This board does not use CONFIG_WDT (DM watchdog support).
Please update the board to use CONFIG_WDT before the
v2019.10 release.
Failure to update by the deadline may result in board removal.
See doc/driver-model/MIGRATION.txt for more info.
====================================================

CONFIG_HW_WATCHDOG is no more a default option for AM33XX devices
after DT/DM conversion, adjusted kconfig accordingly.

DM watchdog support is enabled by default in SPL. The SPL image
doesn't fit into SRAM because of size constraints and build breaks
with an overflow. For this reason DM watchdog support should be
disabled in SPL, driver code should be adjusted accordingly to serve
this purpose.
Built and tested on AM335x device (BeagleboneBlack), compile tested
for all other AM33xx based boards.

Signed-off-by: Suniel Mahesh <sunil.m@techveda.org>