mtd: nand: denali: set DEVICES_CONNECTED 1 if not set

Currently, the driver expects DEVICE_CONNECTED is automatically set
by the hardware, but this feature is disabled in some cases.
In such cases, it is the software's responsibility to set up the
DEVICES_CONNECTED register.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>