devm/simplefb: introduce and use devm_ioremap_wc()

We want to use devm_ioremap_nocache() or even devm_ioremap_wc() to speed
up fbdev writes _a lot_. As devm_ioremap_wc() doesn't exist, yet,
introduce it along the way. Note that ioremap_wc() is aliases to
ioremap_nocache() in asm-generic/{io,iomem}.h so we can safely expect all
architectures to either provide it or use the same alias.

Reported-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>