This commit adds checking whether clock-frequency property acquisition
has succeeded. If not, the frequency is set to 100kHz by default.

The Device Tree binding documentation is updated accordingly.

Based on the intials patches from Zbigniew Bodek
Signed-off-by: NGregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: NZbigniew Bodek <zbb@semihalf.com>
Signed-off-by: NWolfram Sang <wsa@the-dreams.de>