Several DIU registers were being initialized either unnecessarily or to
wrong values.
1) All interrupts were enabled even though there's no interrupt handler.
Interrupts were left enabled when booting Linux.
2) Don't configure a dummy area descriptor, since we don't support ADs
in U-Boot.
3) Don't configure any write-back buffer registers, since we don't use
that mode.
4) The default values for the THRESHOLDS, SYN_POL, and PLUT registers
should be used, so don't touch those registers either.
Signed-off-by: Timur Tabi <timur@freescale.com>
15b83386aa