fpga: dfl: add chardev support for feature devices

For feature devices drivers, both the FPGA Management Engine (FME) and
Accelerated Function Unit (AFU) driver need to expose user interfaces via
the device file, for example, mmap and ioctls.

This patch adds chardev support in the dfl driver for feature devices,
FME and AFU. It reserves the chardev regions for FME and AFU and provide
interfaces for FME and AFU driver to register their device file operations.

Signed-off-by: Tim Whisonant <tim.whisonant@intel.com>
Signed-off-by: Enno Luebbers <enno.luebbers@intel.com>
Signed-off-by: Shiva Rao <shiva.rao@intel.com>
Signed-off-by: Christopher Rauer <christopher.rauer@intel.com>
Signed-off-by: Zhang Yi <yi.z.zhang@intel.com>
Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: Wu Hao <hao.wu@intel.com>
Acked-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>