wdt: aspeed: Add AST2600 watchdog support

AST2600 has 8 watchdog timers including 8 sets of
32-bit decrement counters, based on 1MHz clock.

A 64-bit reset mask is also supported to specify
which controllers should be reset by the WDT reset.

Signed-off-by: Chia-Wei, Wang <chiawei_wang@aspeedtech.com>
Reviewed-by: Ryan Chen <ryan_chen@aspeedtech.com>