arm: remove prototype for udelay_masked

arm: remove prototype for udelay_masked

The interruption support had be removed for ARM architecture and
the function udelay_masked() is no more used except in some timer.c
files  and have the same content than udelay() or __udelay().

This patch update each timer.c implementing this function and
remove the associated prototype in u-boot-arm.h.

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