ast2600: spl: Use readl for reading mmio

u-boot was crashing in qemu as the modeled hardware enforced overly
strict memory reads.

While this code will work on existing hardware, fix to avoid future issues.

Fixes: 12770d0df0e8 ("ast2600: spl: Add boot mode detection")
Link: https://gitlab.com/qemu-project/qemu/-/issues/2636
Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/arch/arm/mach-aspeed/ast2600/spl.c b/arch/arm/mach-aspeed/ast2600/spl.c
index 05390c1..0c5a82e 100644
--- a/arch/arm/mach-aspeed/ast2600/spl.c
+++ b/arch/arm/mach-aspeed/ast2600/spl.c
@@ -45,10 +45,10 @@
 	}
 
 	/* boot from UART has higher priority */
-	if (scu->hwstrap2 & SCU_HWSTRAP2_BOOT_UART)
+	if (readl(&scu->hwstrap2) & SCU_HWSTRAP2_BOOT_UART)
 		return BOOT_DEVICE_UART;
 
-	if (scu->hwstrap1 & SCU_HWSTRAP1_BOOT_EMMC)
+	if (readl(&scu->hwstrap1) & SCU_HWSTRAP1_BOOT_EMMC)
 		return BOOT_DEVICE_MMC1;
 
 out: