alpha: Define ioremap_wc

Commit 3cc2dac5 ("drivers/video/fbdev/atyfb: Replace MTRR UC hole
with strong UC") introduces calls to ioremap_wc and ioremap_uc. This
causes build failures with alpha:allmodconfig. Map the missing functions
to ioremap_nocache.

Fixes: 3cc2dac5

 ("drivers/video/fbdev/atyfb:
        Replace MTRR UC hole with strong UC")
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Luis R. Rodriguez <mcgrof@suse.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Matt Turner <mattst88@gmail.com>