sh: Drop cache flush of the zero page at boot

sh: Drop cache flush of the zero page at boot

SuperH performs cache maintenance on the zero page during boot,
presumably because before commit

  6215d9f4470f ("arch, mm: consolidate empty_zero_page")

the zero page did double duty as a boot params region, and was cleared
separately, as it was not part of BSS. The memset() in question was
dropped by that commit, but the __flush_wback_region() call remained.

As empty_zero_page[] has been moved to BSS, it can be treated as any
other BSS memory, and so the cache flush can be dropped.

Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>