doc: dts: api: remove documentation of chosen `zephyr,ccm`
The `zephyr,ccm` chosen was an ST-specific property which has been replaced
by the generic `zephyr,dtcm`. Remove region initialization code from the
common arch init code (+ BSS zeroing from common XIP init code).
Signed-off-by: Mathieu Choplain <mathieu.choplain-ext@st.com>