Parse FDT to determine PLIC contexts

[calebfletcher]

[calebfletcher]

The interrupts-extended field of the PLIC node specifies the available contexts. The first value in each pair of values in the list is the phandle of the hart's CLINT, and the second value is the interrupt line ID, where -1 means it does not exist, and 9 & 11 refer to supervisor- and machine-mode interrupts.