drm/amd/amdgpu: New debugfs interface for MMIO registers (v5)

This new debugfs interface uses an IOCTL interface in order to pass
along state information like SRBM and GRBM bank switching.  This
new interface also allows a full 32-bit MMIO address range which
the previous didn't.  With this new design we have room to grow
the flexibility of the file as need be.

(v2): Move read/write to .read/.write, fix style, add comment
      for IOCTL data structure

(v3): C style comments

(v4): use u32 in struct and remove offset variable

(v5): Drop flag clearing in op function, use 0xFFFFFFFF for broadcast
      instead of 0x3FF, use mutex for op/ioctl.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>