of: irq: add helper to remap interrupts to another irqdomain

Sometimes interrupts are routed from an interrupt controller to another
in no specific order. Having these in the drives makes it difficult to
maintain when the same drivers supports multiple variants with different
mapping. Also, specifying them in DT makes little sense with a
bunch of numbers like -
	<0, 13>, <5, 32>,
It makes more sense when we can have the parent handle along with
interrupt specifiers for the incoming interrupt as well as that of the
outgoing interrupt like -
	<22 0 &intc 36 0>,
	<24 0 &intc 37 0>,
	<26 0 &intc 38 0>,
And the interrupt specifiers can be interpreted using these properties -
	irqdomain-map-mask = <0xff 0>;
	irqdomain-map-pass-thru = <0 0xff>;

Let's add a helper function to parse this from DT.

Change-Id: Idb3d698ff1d5353d8efc316e21700a1be4ffc542
Patch-mainline: https://lore.kernel.org/patchwork/patch/1026606


Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Lina Iyer <ilina@codeaurora.org>