When building without device tree support (for the mv78x00 SoCs for
example), the build failed because of_irq_count is undeclared. However
mvebu-gpio is not designed to build without device tree support. So
make it depends on OF_CONFIG, remove the #ifdef OF_CONFIG line and the
platform_device_id.

Tested on RD-78x00-mASA, DB-78460-BP, DB-88F6710-BP-DDR3,
DB-MV784MP-GP, Mirabox and OpenBlocks AX3.
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>