evb-ast2600: enabling u-boot-spl feature

Currently, machine evb-ast2600 frozed during u-boot booting.

This patchset has been tested with AST2600-A1 and AST2600-A3 EVB:
AST2600-A1:
```
U-Boot SPL 2019.04 (Jun 16 2021 - 05:59:38 +0000)
already initialized, Trying to boot from RAM

U-Boot 2019.04 (Jun 16 2021 - 05:59:38 +0000)

SOC: AST2600-A1
RST: WDT1 SOC
PCI RST: #2
LPC Mode: SIO:Enable : SuperIO-2e
Eth: MAC0: RGMII, MAC1: RGMII, MAC2: RGMII, MAC3: RGMII
Model: AST2600 EVB
DRAM:  already initialized, 1008 MiB (capacity:1024 MiB, VGA:16 MiB), ECC off
MMC:
sdhci_slot0@100: 1, sdhci_slot1@200: 2, emmc_slot0@100: 0
Loading Environment from SPI Flash... SF: Detected w25q512jv with page size 256 Bytes, erase size 4 KiB, total 64 MiB OK
In:    serial@1e784000
```

AST2600-A3:
```
BP0c00

U-Boot SPL 2019.04 (Jun 16 2021 - 05:59:38 +0000)
already initialized, Trying to boot from RAM

U-Boot 2019.04 (Jun 16 2021 - 05:59:38 +0000)

SOC: AST2600-A3
RST: WDT1 SOC
PCI RST: #2
eSPI Mode: SIO:Enable : SuperIO-2e
Eth: MAC0: RGMII, MAC1: RGMII, MAC2: RGMII, MAC3: RGMII
Model: AST2600 EVB
DRAM:  already initialized, 960 MiB (capacity:1024 MiB, VGA:64 MiB), ECC
off
MMC:

sdhci_slot0@100: 1, sdhci_slot1@200: 2, emmc_slot0@100: 0
Loading Environment from SPI Flash... SF: Detected w25q512jv with page size 256 Bytes, erase size 4 KiB, total 64 MiB OK
In:    serial@1e784000
```

Signed-off-by: Troy Lee <troy_lee@aspeedtech.com>
Change-Id: I77ec3c2928ca96ee3f51290fd3435fca08bb058a
diff --git a/meta-aspeed/conf/machine/evb-ast2600.conf b/meta-aspeed/conf/machine/evb-ast2600.conf
index e011045..d3ee47b 100644
--- a/meta-aspeed/conf/machine/evb-ast2600.conf
+++ b/meta-aspeed/conf/machine/evb-ast2600.conf
@@ -1,6 +1,7 @@
 KERNEL_DEVICETREE = "aspeed-ast2600-evb.dtb"
-UBOOT_MACHINE = "ast2600_openbmc_defconfig"
+UBOOT_MACHINE = "ast2600_openbmc_spl_defconfig"
 UBOOT_DEVICETREE = "ast2600a1-evb"
+SPL_BINARY = "spl/u-boot-spl.bin"
 
 require conf/machine/include/ast2600.inc