bcma: use NS prefix for names of Northstar specific cores

It's cleaner and we don't have quite identical names like
BCMA_CORE_PCIEG2 and BCMA_CORE_PCIE2.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>