riscv: Fix clear bss loop in the start-up code

For RV64, it will use sd instruction to clear t0
register, and the increament will be 8 bytes. So
if the difference between__bss_strat and __bss_end
was not 8 bytes aligned, the clear bss loop will
overflow and acks like system hang.

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: KC Lin <kclin@andestech.com>
Cc: Alan Kao <alankao@andestech.com>
diff --git a/arch/riscv/cpu/u-boot-spl.lds b/arch/riscv/cpu/u-boot-spl.lds
index 32255d5..955dd31 100644
--- a/arch/riscv/cpu/u-boot-spl.lds
+++ b/arch/riscv/cpu/u-boot-spl.lds
@@ -76,7 +76,7 @@
 	.bss : {
 		__bss_start = .;
 		*(.bss*)
-		. = ALIGN(4);
+		. = ALIGN(8);
 		__bss_end = .;
 	} > .bss_mem
 }