mtd: phram: Module parameters add writable permissions

The phram code implements managing multiple devices through a linked
list.
However, due to the module parameter permission of 0, the
/sys/module/phram/parameters/phram interface is missing.
The command line arguments in insmod can only create one device.

Therefore, add writable permissions to the module parameters, create
/sys/module/phram/parameters/phram interface, and create multi-device
support.

Signed-off-by: Xiaoming Ni <nixiaoming@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>