spi_mpc83xx: use brg-frequency for SPI in QE

In case of QE we can use brg-frequency (which is qeclk/2).
Thus no need to divide sysclk in the spi_mpc83xx.

This patch also adds code to use get_brgfreq() on QE chips.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>