ARM: OMAP: Make plat/fpga.h local to arch/arm/plat-omap

There's no need to have this file in plat/fpga.h. We can
make it local to plat-omap replacing fpga_read/write
functions directly with readb/writeb as that's how
they are already defined in fpga.h.

Note that 2420 based H4 is also using the fpga, so let's
keep the led support around in plat-omap until we flip
over mach-omap2 to device tree.

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: linux-fbdev@vger.kernel.org
Cc: Felipe Balbi <balbi@ti.com>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Tony Lindgren <tony@atomide.com>