bcma: get CPU clock

Add method to return the clock of the CPU. This is needed by the arch
code to calculate the mips_hpt_frequency.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>