hw/misc/aspeed_sdmc: Skipping dram_init in u-boot for AST2700

On AST2700 SoC, QEMU now sets BIT6 in VGA0 SCRATCH register to indicate
that DDR training has completed, thus skipping the dram_init().

To align with the recent U-Boot changes, where the Main Control Register's
BIT16 is checked to skip the dram_init() process, this patch sets BIT16 in
the SDMC Main Control Register at reset time.

This allows both the main U-Boot stage to correctly detect and bypass DRAM
initialization when running under QEMU.

Reference:
- QEMU: 2d082fea48
- U-Boot: 94e5435504

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250618080006.846355-2-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

hw/misc/aspeed_sdmc.c

@@ -570,6 +570,9 @@ static void aspeed_2700_sdmc_reset(DeviceState *dev)
     /* Set ram size bit and defaults values */
     s->regs[R_MAIN_CONF] = asc->compute_conf(s, 0);
 
+    /* Skipping dram init */
+    s->regs[R_MAIN_CONTROL] = BIT(16);
+
     if (s->unlocked) {
         s->regs[R_2700_PROT] = PROT_UNLOCKED;
     }