raspberry_pi.c: update platform check for the PI 2 Version B
raspberry_pi.c: update platform check for the PI 2 Version B

The latest Raspberry Pi 2 revision string is missing from the discovery
function mraa_raspberry_pi() and results in incorrect pin mapping defaulting to
PI_B Rev 1.

Signed-off-by: Paul Barrette <5a7dd3512d9575236afc1d4f69696f1b7c77266a@windriver.com>
Signed-off-by: Paul Barrette <b35f8c7746b911163b905ff32288e207dc9b1bbc@gmail.com>
Signed-off-by: Brendan Le Foll <3e0e8c4069934e8b47d99704927a85c66c0ceb33@intel.com>

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