cmd: fdt: Add support for reading stringlist property values

The fdt command currently handles stringlists as strings in 'fdt get value'
subcommand. Since strings in FDT stringlists are separated by '\0', only
the first value gets inserted into the environment variable passed to the
'fdt get value' command.

Example, consider the following DT snippet:

/ { compatible = "foo", "bar" };

The following command only reports the first string in stringlist:
=> fdt get value var / compatible ; print var
foo

It is not possible to assign list of null-terminated strings into U-Boot
environment variable. Add optional 'index' parameter to the subcommand
'fdt get value <var> <path> <prop> [<index>]' which lets user specify which
string within the stringlist should be assigned into the 'var' variable.
The default value of 'index' is 0 in case it is not present. This way the
'fdt' command API does not change and existing scripts are not broken.

The following command now reports the Nth string in stringlist, counting
from zero:
=> fdt get value var / compatible 1 ; print var
bar

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>