pinctrl: pinconf-generic: Fully validate 'pinmux' property

pinctrl: pinconf-generic: Fully validate 'pinmux' property

The pinconf_generic_parse_dt_pinmux() assumes that the 'pinmux' property
is not empty when present. This might be not true. With that, the allocator
will give a special value in return and not NULL which lead to the crash
when trying to access that (invalid) memory. Fix that by fully validating
'pinmux' value, including its length.

Fixes: 7112c05fff83 ("pinctrl: pinconf-generic: Add API for pinmux propertity in DTS file")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>