staging: ste_rmi4: remove cross-dependent platform data

The ux500 machine was actually defining platform data for the
staging driver ste_rmi4, which is not OK. Let us instead define
some __weak platform data in the machine so that the staging
driver can override it at compile-time and we can thus have the
driver self-contained in staging.

Reported-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>