Add support for 4GB boards, splitting and replacing the MACHINE variable
`seco-imx8mm-c61` with `seco-imx8mm-c61-Xgb`, where X can be 2 or 4,
depending on the installed DRAM in the target machine.

Closes https://git.ostc-eu.org/OSTC/planning/hw-enablement/-/issues/59



Signed-off-by: Ettore Chimenti <ettore.chimenti@seco.com>