hw/arm/omap2: Create the RAM in the board

hw/arm/omap2: Create the RAM in the board

The SDRAM is incorrectly created in the OMAP2420 SoC.
Move its creation in the board code, this will later allow the
board to have the QOM ownership of the RAM.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20191021190653.9511-5-philmd@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>