pinctrl: sh-pfc: Improve PINMUX_IPSR_PHYS() documentation

  - The IPSR field is meant for documentation only,
  - The function name refers to the pin function, not to the IPSR
    field.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>