board/BuR/common: fix PMIC mpu-pll setup

If a board-code calls the  pmicsetup(u32 mpupll) with a mpupll value
!= 0 it wants to force some frequency with the value provided by mpupll.
Setting up 1 GHz is wrong here.

Nobody did take notice about that yet, since every board calls this
function with zero.

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>