hw/arm: Allwinner A10 enable SPL load from MMC

hw/arm: Allwinner A10 enable SPL load from MMC

This patch enables copying of SPL from MMC if `-kernel` parameter is not
passed when starting QEMU. SPL is copied to SRAM_A.

The approach is reused from Allwinner H3 implementation.

Tested with Armbian and custom Yocto image.

Signed-off-by: Strahinja Jankovic <strahinja.p.jankovic@gmail.com>
Reviewed-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Message-id: 20221226220303.14420-7-strahinja.p.jankovic@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>