configs: stm32mp15: move bootdelay configuration in defconfig

The STM32MP15 boards have no reason to configure bootdelay in stm32mp1.h
as it is already done with CONFIG_BOOTDELAY (default = 2) and in
include/env_default.h.

This patch allows configuration for customers which reuse stm32mp1.h
and reduce the size of the default environment.

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