drm/ast: Use constants for WDT registers

drm/ast: Use constants for WDT registers

WDT is the Watchdog timer. WDC registers are located at the memory
range at [0x1e785000, 0x1e785fff]. There are currently up to 8
watchdog timers in the range, of which 2 are being used by AST2500.

Refer to them with macros named AST_REG_WDT<n>, where <n> is the byte
offset into the watchdog timer's memory range. Each macro also takes
the index of the watchdog timer to access. A new watchdog timer starts
at each 0x40 byte offset.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patch.msgid.link/20260327133532.79696-8-tzimmermann@suse.de