Move stm32mp15_dhcom_basic_defconfig into stm32mp15_dhcom_basic.config.
Retain legacy stm32mp15_dhcom_basic_defconfig as multi-config for all
DH STM32MP15xx DHCOM based boards. Move stm32mp15_dhsor_basic_defconfig
into stm32mp15_dhsor_basic.config. Retain stm32mp15_dhsor_basic_defconfig
as multi-config for all DH STM32MP15xx DHCOR based boards.

Introduce separate stm32mp15_dhcom_drc02_basic_defconfig,
stm32mp15_dhcom_pdk2_basic_defconfig, stm32mp15_dhcom_picoitx_basic_defconfig
for each STM32MP15xx DHCOM based board and separate
stm32mp15_dhcor_avenger96_basic_defconfig,
stm32mp15_dhcor_drc_compact_basic_defconfig,
stm32mp15_dhcor_testbench_basic_defconfig for each
STM32MP15xx DHCOR based board, to make build for those boards easier.
No functional change.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>