hw/misc/aspeed_scu: Skipping dram_init in u-boot

hw/misc/aspeed_scu: Skipping dram_init in u-boot

Setting BIT6 in VGA0 SCRATCH register will indicate that the ddr traning
is done, therefore skipping the u-boot-spl dram_init() process.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Signed-off-by: Troy Lee <troy_lee@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Tested-by: Nabih Estefan <nabihestefan@google.com>
Link: https://lore.kernel.org/qemu-devel/20250304064710.2128993-2-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>