mtd: phram: Add the kernel lock down check

The phram MTD driver may map any memory pages no matter whether it's
reserved or whatever used for systems, which basically allows user
bypassing the lock down.

Add the check and abort the probe if the kernel is locked down for
LOCKDOWN_DEV_MEM.

Reported-by: Fabian Vogt <fvogt@suse.com>
Suggested-by: Fabian Vogt <fvogt@suse.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>