configs: stm32mp1: Remove misleading CONFIG_SPL_BSS_START_ADDR

CONFIG_SPL_BSS_START_ADDR is only used on a few mach- linker scripts.
stm32mp1 uses the generic script under arch/arm/cpu/u-boot-spl.lds,
which does not make use of this definition.

The SPL BSS starts in SRAM, right after .text, .rodata, .data, and
.u_boot_list. A very short version of the STM32MP1 memory map is:
  * SYSRAM: 2ffc0000 - 30000000     <- all of SPL is here
  * DRAM:   c0000000+

0xC0200000 is a DRAM address, and has nothing to do with SPL. It is
just very misleading to have it next to CONFIG_SPL_BSS_MAX_SIZE, or to
have it at all.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>