drm/i915/gt: Expose per-gt RPS defaults in sysfs

Add the following sysfs files to gt/gtN/.defaults/:
* rps_min_freq_mhz
* rps_max_freq_mhz

v2: Correct gt/gtN/.defaults/* file names in commit message
v3: Remove rps_boost_freq_mhz since it is not consumed by userspace

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/cf6e483bf79f871c2c8c74af6005bf6a83a3a1ce.1658192398.git.ashutosh.dixit@intel.com