arc: axs103_smp: Fix CPU frequency to 100MHz for dual-core

The most recent release of AXS103 [v1.1] is proven to work
at 100 MHz in dual-core mode so this change uses mentioned feature.
For that we:
 * Update axc003_idu.dtsi with mention of really-used CPU clock freq
 * Remove clock override in AXS platform code for dual-core HW

Note we're still leaving a hack for clock "downgrade" on early boot
for quad-core hardware.

Also note this change will break functionality of AXS103 v1.0 hardware.
That means all users of AXS103 __must__ upgrade their boards with the
most recent firmware.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>