simplefb: use write-combined remapping

Framebuffers shouldn't be cached and it is usually very uncommon to read
them. Therefore, use ioremap_wc() to get significant speed improvements on
systems which provide it. On all other systems it's aliased to
ioremap_nocache() which is also fine.

Reported-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Tested-by: Tom Gundersen <teg@jklm.no>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>