meta-bletchley: chassis: set OBMC_CHASSIS_INSTANCES

Bletchley has the primary sled and N secondary slots, each of which
should be considered a 'chassis' from a power control perspective.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I9f948c31c720e13c72a9f4d68500fd8990a4849b