drm/i915/slpc: Add sysfs for SLPC power profiles

drm/i915/slpc: Add sysfs for SLPC power profiles

Default SLPC power profile is Base(0). Power Saving mode(1)
has conservative up/down thresholds and is suitable for use with
apps that typically need to be power efficient.

Selected power profile will be displayed in this format-

$ cat slpc_power_profile

  [base]    power_saving

$ echo power_saving > slpc_power_profile
$ cat slpc_power_profile

  base    [power_saving]

v2: Disable waitboost in power saving profile, update sysfs
format and add some kernel doc for SLPC (Rodrigo)

v3: Update doc with info about power profiles (Rodrigo)

v4: Checkpatch warning and remove extra line (Rodrigo)

Cc: Sushma Venkatesh Reddy <sushma.venkatesh.reddy@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250117215753.749906-1-vinay.belgaumkar@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>