[BHND/bcma] Add implementation of BHND_BUS_RESET_CORE function for BCMA

This patch addes missing implementation of BHND_BUS_RESET_CORE function for BCMA.
The reset procedure is very simple: enable reset mode, stop clocking,
enable clocking & force clock gating, disable reset mode, stop clock gating.

Tested:

• (michael) Tested on ASUS RT-N53 for enabling/reset USB core

Submitted by: Michael Zhilin <mizhka@gmail.com>
Approved by: re (gjb)