riscv: remove redundant mv instructions

Some mv instructions were useful when first introduced to preserve a0 and
a1 before function calls. However the code has changed and they are now
redundant. Remove them.
Signed-off-by: Nam Cao <namcaov@gmail.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20230725053835.138910-1-namcaov@gmail.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/head.S

@@ -289,10 +289,6 @@ clear_bss:
 	blt a3, a4, clear_bss
 clear_bss_done:
 #endif
-	/* Save hart ID and DTB physical address */
-	mv s0, a0
-	mv s1, a1
-
 	la a2, boot_cpu_hartid
 	XIP_FIXUP_OFFSET a2
 	REG_S a0, (a2)
@@ -306,7 +302,7 @@ clear_bss_done:
 	la a0, __dtb_start
 	XIP_FIXUP_OFFSET a0
 #else
-	mv a0, s1
+	mv a0, a1
 #endif /* CONFIG_BUILTIN_DTB */
 	call setup_vm
 #ifdef CONFIG_MMU