cmd: fdt: Add support for integer arrays in fdt get value with index

Currently any integer array value is set as long up-to-40 character
hexadecimal string into environment variable when extracted from an
FDT using 'fdt get value path prop index', because the support for
handling integer arrays is not implemented, and fdt_value_env_set()
code falls back into the hash handling behavior instead.

Implement this support simply by checking whether user supplied any
index. If index is set and the property length is multiple of four,
then this is an integer array, and the code would extract value at
specified index.

There is a subtle change where default index is set to -1 instead of 0.
This is OK, since the only place which checks for index to be less or
equal zero is the string array handling code in fdt_value_env_set() and
that code would work perfectly well with index -1 too.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>