meta-openpower: Define i2c-occ feature

OpenPOWER P8 systems uses i2c to access occ.
Define i2c-occ OBMC_MACHINE_FEATURES and append EXTRA_OECONF in
openpower-occ-control.bb so that it compiles with I2C OCC support.

Enabled this feature on Palmetto first.

Change-Id: I1c07d28494fc5e87e86a3be1bfcfd955770e14e7
Signed-off-by: Lei YU <mine260309@gmail.com>
diff --git a/conf/machine/include/p8.inc b/conf/machine/include/p8.inc
new file mode 100644
index 0000000..b43045d
--- /dev/null
+++ b/conf/machine/include/p8.inc
@@ -0,0 +1 @@
+OBMC_MACHINE_FEATURES += 'i2c-occ'