clocksource: fixup ux500 build problems

Based on a patch from Arnd Bergmann this fixes up the build
problem of assigning a non-existing global when the ux500 PRCMU
timer is not linked in by passing its base address to the init
function. We also add a missing <linux/errno.h> inclusion and
staticize the dummy function.

Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>