boards: add missing ram/flash entries to board .yaml

Boards used for testing need to specify the available ram and
flash to be effectively filtered. If the entries are missing,
they default to 0, so any test which specifies minimum
ram or flash excludes them. This is especially an issue if
a board is specified as an integration platform.

Added entries for:

- qemu_cortex_a53
- qemu_xtensa_dc233c
- mps2_an385

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>