sh: boot: Remove sh5 cache handling

Commit 37744fee ("sh: remove sh5 support") in v5.8 forgot to
remove the sh5 cache handling.

Suggested-by: Yoshinori Sato <ysato@users.sourceforge.jp>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/23e9b3fd0d78e46c9fc1835852ba226aba92c3ca.1713959531.git.geert+renesas@glider.be


Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>