arm: remove prototype for get_timer_masked

arm: remove prototype for get_timer_masked

The interruption support had be removed for ARM architecture and
the function get_timer_masked() is no more used except in some
the timer.c files.

This patch clean each timer.c which implement this function and
remove the associated prototype in u-boot-arm.h

For timer.c, I don't verify if the weak version of get_timer
(in lib/time.c) can be used

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>