aspeed: add a max_ram_size property to the memory controller

This will be used to construct a memory region beyond the RAM region
to let firmwares scan the address space with load/store to guess how
much RAM the SoC has.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Tested-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20180807075757.7242-7-joel@jms.id.au
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>