of: Provide a function to request and map memory

A call to of_iomap does not request the memory region. This patch adds the
function of_io_request_and_map which requests the memory region before
mapping it.

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Suggested-by: Rob Herring <robh@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>