watchdog: dw_wdt: Support devices with non-fixed TOP values

In case if the DW Watchdog IP core is synthesised with
WDT_USE_FIX_TOP == false, the TOP interval indexes make the device
to load a custom periods to the counter. These periods are hardwired
at the IP synthesis stage and can be within [2^8, 2^(WDT_CNT_WIDTH - 1)].
Alas their values can't be detected at runtime and must be somehow
supplied to the driver so one could properly determine the watchdog
timeout intervals. For this purpose we suggest to have a vendor-
specific dts property "snps,watchdog-tops" utilized, which would
provide an array of sixteen counter values. At device probe stage they
will be used to initialize the watchdog device timeouts determined
from the array values and current clocks source rate.

In order to have custom TOP values supported the driver must be
altered in the following way. First of all the fixed-top values
ready-to-use array must be determined for compatibility with currently
supported devices, which were synthesised with WDT_USE_FIX_TOP == true.
It will be used if either fixed TOP feature is detected being enabled or
no custom TOPs are fetched from the device dt node. Secondly at the probe
stage we must initialize an array of the watchdog timeouts corresponding
to the detected TOPs list and the reference clock rate. For generality the
procedure of initialization is designed in a way to support the TOPs array
with no limitations on the items order or value. Finally the watchdog
period search methods should be altered to support the new timeouts data
structure.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: devicetree@vger.kernel.org
Link: https://lore.kernel.org/r/20200530073557.22661-5-Sergey.Semin@baikalelectronics.ru


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>