Adds support for SDRAM via `SEMC` peripheral. SDRAM support can be
enabled in the mpconfigboard.mk file by setting `MICROPY_HW_SDRAM_AVAIL`
to `1` and poviding the size of the RAM via `MICROPY_HW_FLASH_SIZE`.
When SDRAM support is enabled the whole SDRAM is currently used used
for MicroPython heap.
Signed-off-by: Philipp Ebensberger
