dts: riscv32: rv32m1: Disable unused interrupt multiplexers

Commit 948ef47c ("dts: riscv32: Add rv32m1 zero-riscy core") put
common ri5cy and zero_riscy DT parts in rv32m1.dtsi, with two separate
rv32m1_ri5cy.dtsi rv32m1_zero_riscy.dtsi files that #include it.

The two interrupt multiplexers are defined in the common rv32m1.dtsi
file, but rv32m1_ri5cy.dtsi rv32m1_zero_riscy.dtsi each only specify
'interrupts' for one of them. Since 'interrupts' is 'category: required'
in openisa,rv32m1-intmux.yaml, this leads to warnings (or errors with
the new DT parser).

Disable (status = "disabled") the two interrupt multiplexers in the
common rv32m1.dtsi file and enable them in the board-specific files to
fix it. Required props. are only checked for enabled nodes.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>