configs: stm32mp15: enable LOG features

Activate the logging features in SPL and U-Boot
(CONFIG_LOG and CONFIG_SPL_LOG) and the command 'log' with
CONFIG_CMD_LOG.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>