driver core: Const-correct platform getbyname functions

This converts resource and IRQ getbyname functions for the platform
bus to use const char *, I ran into compiler moanings when I tried
using a const char * for looking up a certain resource.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>