hw/arm/imx25_pdk: Fix error message for invalid RAM size

The i.MX25 PDK board has 2 banks for SDRAM, each can
address up to 256 MiB. So the total RAM usable for this
board is 512M. When we ask for more we get a misleading
error message:

  $ qemu-system-arm -M imx25-pdk -m 513M
  qemu-system-arm: Invalid RAM size, should be 128 MiB

Update the error message to better match the reality:

  $ qemu-system-arm -M imx25-pdk -m 513M
  qemu-system-arm: RAM size more than 512 MiB is not supported

Fixes: bf350daa ("arm/imx25_pdk: drop RAM size fixup")
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-id: 20210407225608.1882855-1-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>