docs: update C61 build instructions

C61 build has been split up from the target MACHINE=seco-imx8mm-c61 to
two targets, MACHINE=seco-imx8mm-c61-2gb and MACHINE=seco-imx8mm-c61-4gb,
depending on the memory on the card. Reflect that in the build
instructions.

Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>
Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>