There is no need to acquire the spinlock when reading from a pin
controller register:
  1. Reading a single MMIO register is an atomic operation,
  2. While sh_pfc_phys_to_virt() inside sh_pfc_read() has to traverse
     all mapped windows to find the appropriate virtual address, this
     does not need any locking, as the window mappings are never
     changed.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/2008ca99f0079fd5d4e640b7ef78710c98cc9f77.1637143108.git.geert+renesas@glider.be