hw/misc/aspeed_sdmc: Fix incorrect memory size

hw/misc/aspeed_sdmc: Fix incorrect memory size

The SDRAM Memory Controller has a 32-bit address bus, thus
supports up to 4 GiB of DRAM. There is a signed to unsigned
conversion error with the AST2600 maximum memory size:

  (uint64_t)(2048 << 20) = (uint64_t)(-2147483648)
                         = 0xffffffff40000000
                         = 16 EiB - 2 GiB

Fix by using the IEC suffixes which are usually safer, and add
an assertion check to verify the memory is valid. This would have
caught this bug:

  $ qemu-system-arm -M ast2600-evb
  qemu-system-arm: hw/misc/aspeed_sdmc.c:258: aspeed_sdmc_realize: Assertion `asc->max_ram_size < 4 * GiB' failed.
  Aborted (core dumped)

Fixes: 1550d72679 ("aspeed/sdmc: Add AST2600 support")
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>