mtd: physmap_of: use OF helpers for reading strings

OF core code provides helpers for counting strings and reading them so
use them instead of doing this manually. This simplifies the code a bit.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>