cmd: fdt: Import is_printable_string() from DTC to fix u32 misprint

cmd: fdt: Import is_printable_string() from DTC to fix u32 misprint

Import is_printable_string() implementation from DTC 1.7.0 as of
DTC commit 039a994 ("Bump version to v1.7.0") . This fixes a print
of u32 property which so far used to be printed as string by U-Boot
fdt print command.

We might see the case where the parsed property value, in this case
it is a 32-bit integer, identified as a printable string or a null byte
(concatenated strings) because of its last character happens to be:
  0x00 (null character), 0xB (vertical tab character) or
  0x10 (line feed character)
In this situation, if the string is identified as printable string,
it will be displayed as character instead of hex value

When the isprint() condition is true, there are two possibilities:
  1) The character is ASCII character (except the first 32)
  2) The character is extended ASCII character

For example,
NG property in device tree:
    clock-frequency = <16640000>;
by default, would be displayed as
    clock-frequency = "", "ýè";
and with this patch applied, would be displayed as
    clock-frequency = <0x00fde800>;

Full investigation was done by Nam and Hai, patch reworked by Marek
to use common code from DTC.

Signed-off-by: Hai Pham <hai.pham.ud@renesas.com>
Signed-off-by: Nam Nguyen <nam.nguyen.yh@renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>