watchdog: armada_37xx: Add missing dependency on HAS_IOMEM

The following kbuild warning is seen on a system without HAS_IOMEM.

WARNING: unmet direct dependencies detected for MFD_SYSCON
  Depends on [n]: HAS_IOMEM [=n]
  Selected by [y]:
  - ARMADA_37XX_WATCHDOG [=y] && WATCHDOG [=y] && (ARCH_MVEBU || COMPILE_TEST

This results in a subsequent compile error.

drivers/watchdog/armada_37xx_wdt.o: in function `armada_37xx_wdt_probe':
armada_37xx_wdt.c:(.text+0xdc): undefined reference to `devm_ioremap'

Add the missing dependency.

Reported-by: Necip Fazil Yildiran <fazilyildiran@gmail.com>
Fixes: 54e3d9b5 ("watchdog: Add support for Armada 37xx CPU watchdog")
Link: https://lore.kernel.org/r/20201108162550.27660-1-linux@roeck-us.net


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>