board: ti: x15: Add support for beagle_X15 revC

BeagleBoard X15 revC board is similar to X15 revB1 except
with a SR2.0 where revB1 uses a SR1.1. Add board detection
support for revC.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>