of/base: Take NULL string into account for property with multiple strings

of/base: Take NULL string into account for property with multiple strings

The current implementation just ignore any NULL string inserted in a
multiple strings property.
In some cases we can have a property with a fix number of strings but
not necessarily used, like for example in a list of valid pinmux modes.

 prop = "uart_rx", "uart_tx", "", "", "safe_mode";

Do no skip NULL string and take them into account in
of_property_read_string_index and of_property_count_strings.

Reported-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>