powerpc/86xx: improve calculation of DIU pixel clock on the MPC8610 HPCD

mpc8610hpcd_set_pixel_clock() calculates the correct value of the PXCLK
bits in the CLKDVDR register for a given pixel clock rate.  The code which
performs this calculation is overly complicated and includes an error
estimation routine that doesn't work most of the time anyway.  Replace the
code with the simpler routine that's currently used on the P1022DS.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>