board: stm32mp1: move CONFIG_ENV_XXX in defconfig

Move CONFIG_ENV_SECT_SIZE and CONFIG_ENV_OFFSET in stm32mp15_*_defconfig
for ST board with NOR support (STM32MP15xx-EV1 boards)
- CONFIG_SECT_SIZE values = the max supported NOR erase size (256KB)
- CONFIG_ENV_OFFSET = offset for NOR (ENV_IS_IN_SPI_FLASH)

This Patch prepares the U-Boot support of boards with STM32MP15x SOC
not provided by STMicroelectronics.

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