fs: nvs: introduce nvs_mount and deprecate nvs_init

Add a new API call to replace nvs_init: nvs_mount. The new API does the
same as nvs_init except that it assumes to be provided with a valid
flash device via `struct nvs_fs` `flash_device` field. Previously, it
was not possible to avoid the runtime overhead of device_get_binding()
even if the flash device was known at compile time.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>