arm: armv7: solve issue for timer_rate_hz in arch timer

arm: armv7: solve issue for timer_rate_hz in arch timer

The current value timer_rate_hz causes a problem with function
timer_get_us() from lib time and then an issue with
readx_poll_timeout() function.

With corrected value for tbclk() = timer_rate_hz = CONFIG_SYS_HZ_CLOCK
the weak functions in lib timer can be used:
- get_timer()
- __udelay()
So the specific function in this file are removed.

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