power: reset: reboot-mode: fix build error of missing ioremap/iounmap on UM

commit 4fcd504e

 ("power: reset: add reboot mode driver") uses api from
syscon, and syscon uses ioremap/iounmap which depends on HAS_IOMEM, so
let's depend on MFD_SYSCON instead of selecting it directly to avoid the
um-allyesconfig like build error on archs that without iomem:

drivers/mfd/syscon.c: In function 'of_syscon_register':
drivers/mfd/syscon.c:67:9: error: implicit declaration of function 'ioremap' [-Werror=implicit-function-declaration]
  base = ioremap(res.start, resource_size(&res));
         ^
drivers/mfd/syscon.c:67:7: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
  base = ioremap(res.start, resource_size(&res));
       ^
drivers/mfd/syscon.c:109:2: error: implicit declaration of function 'iounmap' [-Werror=implicit-function-declaration]
  iounmap(base);
  ^

Reported-by: Kbuild test robot <fengguang.wu@intel.com>
Fixes: 4fcd504e

("power: reset: add reboot mode driver")
Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>