microblaze: fix build failure due to undefined reference to `get_ticks'

after commit "common: add possibility for readline_into_buffer
timeout" (sha1:9c348311) was applied. The Microblaze generic build
fails with error below:

common/libcommon.o: In function `cread_line':
/devel/u-boot/common/main.c:717: undefined reference to `get_ticks'
/devel/u-boot/common/main.c:717: undefined reference to `get_tbclk'
/devel/u-boot/common/main.c:720: undefined reference to `get_ticks'

Signed-off-by: Stephan Linz <linz@li-pro.net>
Acked-by: Michal Simek <monstr@monstr.eu>