[WATCHDOG] orion5x_wdt.c: 'ORION5X_TCLK' undeclared

orion5x_wdt no longer compiled after the changes in commit
ebe35aff

 ("Orion: prepare for
runtime-determined timer tick rate").

The tick rate define (ORION5X_TCLK) was removed in favor of a runtime
detection. The quick fix is to add the define in the watchdog driver.
The fix is not correct for all supported orion5x platforms, but since
the supported platforms right now are 133 Mhz and 166 Mhz, it won't
be _that_ far off. ;-) A fix that uses the runtime-determined timer
tick rate will be applied later.

Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Cc: Kristof Provost <kristof@sigsegv.be>
Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>
Cc: Nicolas Pitre <nico@cam.org>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>