mfd: db8500 stop passing around platform data

Except for the constant DB8500_PRCMU_FW_VERSION_OFFSET number, nothing
is ever passed through the platform data and used in a driver, so we
can simply stop passing it around.

Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>