fix(st): manage UART clock and reset only in BL2

fix(st): manage UART clock and reset only in BL2

As the UART is already initialized, no need to check for UART clock
or reset in next BL. An issue can appear if the next BL device tree
(e.g HW_CONFIG) doesn't use the same clocks or resets (like SCMI ones).

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I044ef2386abe2d3dba5a53c3685440d64ca50a4f