watchdog: at91sam9: at91_wdt_dt_ids cannot be __init

The device IDs are referenced by the driver and potentially
used beyond the init time, as kbuild correctly warns
about. Remove the __initconst annotation.

Without this patch, building at91_dt_defconfig results in:

WARNING: drivers/watchdog/built-in.o(.data+0x28): Section mismatch in reference from the variable at91wdt_driver to the (unknown reference) .init.rodata:(unknown)
The variable at91wdt_driver references
the (unknown reference) __initconst (unknown)

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Tested-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Cc: linux-watchdog@vger.kernel.org