bcma: implement setting core clock mode to dynamic

This patch is based on code from _ai_clkctl_cc() in brcmsmac.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Tested-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>