Revert "ANDROID: GKI: Export clocksource_mmio_init"

This reverts commit c0b208db.

The symbols exported by it (clocksource_mmio_init and
clocksource_mmio_readl_up) are not used by any vendor symbol list, and
so the exports are not needed.

Bug: 161675989
Cc: Chun-Hung Wu <chun-hung.wu@mediatek.com>
Change-Id: Ic3b1105d093815ead780519661184aeab9c1f026
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>