Change a maximum size of URI format property to 256 octets

Change a maximum size of URI format property to 256 octets

As per OCF specification, a normal string-type property without
any statement for maximum length can have a 64 octets size at maximum.
However, for some cases, e.g. a string for URL, one can use longer string
value with a explicit statement of length in its resource model.
Fortunately, url format is already defined in OCF spec. as one of property type,
which can have 256 octets at maximum.

Change-Id: Ia8536ced5f4ab71ccdabd98e9936b00513efaa27
Signed-off-by: Jihun Ha <jihun.ha@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/17457
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Heewon Park <h_w.park@samsung.com>
Reviewed-by: Uze Choi <uzchoi@samsung.com>