timer: sti: convert sti-timer to arm a9 global timer

STI timer is actually ARM Cortex A9 global timer. Convert the driver to
use generic global timer name and make it consistent with Linux kernel
global timer driver. This also allows any A9 based device to use this
driver.

Signed-off-by: William Zhang <william.zhang@broadcom.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Tested-by: Patrice Chotard <patrice.chotard@foss.st.com>