stm32mp: remove redundant SYS_TEXT_BASE prompt

The main prompt for this (defined in /Kconfig) is visible at all times,
which means there's no reason to have an additional, machine-specific
prompt to set the same option.

Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>