firmware: Simplify FDT header endianness conversion

Reduce the code size by using single-byte loads instead of bit
manipulation. This method also does the right thing on (hypothetical)
big-endian systems.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

firmware/fw_base.S

@@ -242,30 +242,14 @@ _scratch_init:
 	beq	t1, a1, _fdt_reloc_done
 	/* t0 = source FDT start address */
 	add	t0, a1, zero
-	/* t2 = source FDT size in big-endian */
-#if __riscv_xlen > 32
-	lwu	t2, 4(t0)
-#else
-	lw	t2, 4(t0)
-#endif
-	/* t3 = bit[15:8] of FDT size */
-	add	t3, t2, zero
-	srli	t3, t3, 16
-	and	t3, t3, a4
+	/* t2 = source FDT size (convert from big-endian) */
+	lbu	t2, 7(t0)
+	lbu	t3, 6(t0)
+	lbu	t4, 5(t0)
+	lbu	t5, 4(t0)
 	slli	t3, t3, 8
-	/* t4 = bit[23:16] of FDT size */
-	add	t4, t2, zero
-	srli	t4, t4, 8
-	and	t4, t4, a4
 	slli	t4, t4, 16
-	/* t5 = bit[31:24] of FDT size */
-	add	t5, t2, zero
-	and	t5, t5, a4
 	slli	t5, t5, 24
-	/* t2 = bit[7:0] of FDT size */
-	srli	t2, t2, 24
-	and	t2, t2, a4
-	/* t2 = FDT size in little-endian */
 	or	t2, t2, t3
 	or	t2, t2, t4
 	or	t2, t2, t5