dts: bindings: memc: stm32: fmc: add st,mem-swap

Introduces H7 FMC bindings with support for
memory remap or swap configuration.

The following values are supported:

* disabled - default mapping (reset state).
* sdram-sram - swaps the NOR/PSRAM and SDRAM banks.
* sdramb2 - remaps SDRAM bank 2.

Signed-off-by: Georgij Cernysiov <geo.cgv@gmail.com>