fbdev: arm has __raw I/O accessors, use them in fb.h

This removes the sparse warnings on arm platforms:

warning: cast removes address space of expression

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: H Hartley Sweeten <hsweeten at visionengravers.com>
Cc: Alexander Shiyan <shc_work@mail.ru>
Cc: Russell King <linux@arm.linux.org.uk>