fpga: add method to get fpga manager from device

The intent is to provide a non-DT method of getting
ahold of a FPGA manager to do some FPGA programming.

This patch refactors of_fpga_mgr_get() to reuse most of it
while adding a new method fpga_mgr_get() for getting a
pointer to a fpga manager struct, given the device.

Signed-off-by: Alan Tull <atull@opensource.altera.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>