i2c: stu300: use devm managed resources

Allocate memory for device state using devm_kzalloc(), get the
clock using devm_clk_get(), get the IRQ using devm_request_irq(),
request and remap memory using devm_request_and_ioremap().
All to simplify accounting and letting the kernel do the
garbage-collection.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>