soc: linker.ld: add linker script for stm32mp2x

Add a linker script for the stm32mp2x soc series. It includes the
standard arm cortex-m linker and adds standard zephyr relocation
sections.
Replace the rom_start section name with .isr_vectors in the linker
script. This is necessary for the zephyr firmware to be started by the
remote proc driver which expects the section containing the vector table
to be named .isr_vectors.

Signed-off-by: Youssef Zini <youssef.zini@savoirfairelinux.com>