fpga: dfl: add feature device infrastructure

fpga: dfl: add feature device infrastructure

This patch abstracts the common operations of the sub features and defines
the feature_ops data structure, including init, uinit and ioctl function
pointers. And this patch adds some common helper functions for FME and AFU
drivers, e.g. dfl_feature_dev_use_begin/end which are used to ensure
exclusive usage of the feature device file.

Signed-off-by: Tim Whisonant <[email protected]>
Signed-off-by: Enno Luebbers <[email protected]>
Signed-off-by: Shiva Rao <[email protected]>
Signed-off-by: Christopher Rauer <[email protected]>
Signed-off-by: Kang Luwei <[email protected]>
Signed-off-by: Zhang Yi <[email protected]>
Signed-off-by: Xiao Guangrong <[email protected]>
Signed-off-by: Wu Hao <[email protected]>
Acked-by: Alan Tull <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>