libsel4vm: fix GIC_DIST_CTLR enable handling

[axel-h]

We have seen that some VMs try to write 0x03 instead of 0x01. Support this case more gracefully and ensure that the GIC_DIST_CTLR_EN_GRP1 setting properly applied.

• check the bit an not the value
• ignore GIC_DIST_CTLR_EN_GRP1 and print a warning
• print the value if unknown bits are set