If the board defines `FMC_RAM_ADDR` and `FMC_RAM_LEN`, the FMC RAM is used a additional heap if module `periph_fmc` is enabled.
For that purpose
- the linker symbols `_fmc_ram_addr` and `_fmc_ram_len` are set,
- a memory region `fcmram` is added in linker script for the FMC RAM based on these `_fcm_ram_*` linker symbols
- a section for the FMC RAM is defined in this memory region that defines the heap by setting `_sheap3` and `_eheap3` and
- the number of heaps is set to 4 since to use `_sheap3` and `_eheap3` even though `_sheap1` and `_eheap1` (the backup RAM) and `_sheap2` and `_eheap2` (SRAM4) are not present.
