configs: stm32mp: cleanup the stm32mp15 file

Remove STM32_SYSRAM_END and clean the comments in stm32mp15_common.h file
after moving some CONFIG to Kconfig: CONFIG_SYS_CBSIZE,
CONFIG_SPL_MAX_FOOTPRINT, CONFIG_SYS_SPL_MALLOC_START and
CONFIG_SYS_SPL_MALLOC_SIZE.

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