arm: at91: Remove CONFIG_AT91_HW_WDT_TIMEOUT

This patch removes the CONFIG_AT91_HW_WDT_TIMEOUT as its not needed any
more. The WD timeout value can be provided via the "timeout-sec" DT
property. If not provided this way, the default value of 2 seconds will
be used.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Andreas Bießmann <andreas@biessmann.org>
Cc: Eugen Hristev <eugen.hristev@microchip.com>