riscv: ae250: Support DT provided by the board at runtime

Enable CONFIG_OF_BOAD to support delivery dtb to u-boot
at run time instead of embedded.

There are two methods to delivery dtb.
 1 Pass from loader:
   When u-boot boot from RAM, gdb or loader can pass dtb
   via a2 to u-boot dynamically. Of course gdb or loader
   shall be in charge of dtb delivery.

 2 Configure CONFIG_SYS_FDT_BASE:
   It can be configured as RAM or ROM base statically,
   no mater u-boot boot from RAM or ROM.
   If it was configured as ROM base, dtb can be burned
   into ROM(spi flash) by spi driver.

Meanwhile remove CONFIG_SKIP_LOWLEVEL_INIT which is
useless in nx25-ae250 configuration.

Signed-off-by: Rick Chen <rick@andestech.com>
Signed-off-by: Rick Chen <rickchen36@gmail.com>
Cc: Greentime Hu <green.hu@gmail.com>

arch/riscv/cpu/nx25/start.S

@@ -45,6 +45,8 @@ trap_vector:
 
 .global trap_entry
 handle_reset:
+	li t0, CONFIG_SYS_SDRAM_BASE
+	SREG a2, 0(t0)
 	la t0, trap_entry
 	csrw mtvec, t0
 	csrwi mstatus, 0

board/AndesTech/nx25-ae250/nx25-ae250.c

@@ -64,3 +64,12 @@ ulong board_flash_get_legacy(ulong base, int banknum, flash_info_t *info)
 {
 	return 0;
 }
+
+void *board_fdt_blob_setup(void)
+{
+	void **ptr = (void *)CONFIG_SYS_SDRAM_BASE;
+	if (fdt_magic(*ptr) == FDT_MAGIC)
+			return (void *)*ptr;
+
+	return (void *)CONFIG_SYS_FDT_BASE;
+}

configs/nx25-ae250_defconfig

@@ -16,6 +16,7 @@ CONFIG_CMD_CACHE=y
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
 CONFIG_OF_CONTROL=y
+CONFIG_OF_BOARD=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_DM=y

include/configs/nx25-ae250.h

@@ -11,18 +11,9 @@
 /*
  * CPU and Board Configuration Options
  */
-#define CONFIG_SKIP_LOWLEVEL_INIT
-
 #define CONFIG_BOOTP_SEND_HOSTNAME
 #define CONFIG_BOOTP_SERVERIP
 
-#ifdef CONFIG_SKIP_LOWLEVEL_INIT
-#ifdef CONFIG_OF_CONTROL
-#undef CONFIG_OF_SEPARATE
-#define CONFIG_OF_EMBED
-#endif
-#endif
-
 /*
  * Miscellaneous configurable options
  */
@@ -50,6 +41,9 @@
  */
 #define CONFIG_SYS_MALLOC_LEN   (512 << 10)
 
+/* DT blob (fdt) address */
+#define CONFIG_SYS_FDT_BASE		0x000f0000
+
 /*
  * Physical Memory Map
  */