include/arch/arm: linker.ld: Add shared memory sections definitions

STM32WB HCI driver requires definition of 2 RAM regions to support
use of 3 shared memory sections: MAPPING_TABLE, MB_MEM1 and MB_MEM2.
In linker.ld, under conditions of HCI driver to be enabled,
define SRAM1 and SRAM2 based on input defined in stm32wb linker.
Then define the 3 sections MAPPING_TABLE, MB_MEM1 and MB_MEM

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>