drivers/block/z2ram: use ioremap_wt() instead of __ioremap(_PAGE_WRITETHRU)

_PAGE_WRITETHRU is a target specific flag. Prefer generic functions.

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>