microblaze: add parameter to microblaze_read()

This patch modifies parameter of microblaze_read() from 'void' to 'struct
clocksource *cs', which fixes compile warning for incompatible parameter type.

Signed-off-by: Coly Li <coly.li@suse.de>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Michal Simek <monstr@monstr.eu>