libplatsupport: fix previous reverts

Due to my messing up of changelists, the revert
did not go through as cleanly as I'd hoped.

This commit reinstates
"libplatsupport: riscv plic irqchip parser"
23092562396ae9209679c83d75d25a953b70c2c6
and picks changes necessary from
"libplatsupport: hifive ltimer refactor"
c0c8ddcfac95a09b50e82cfa88367145d7aeaa5e
in order to build
"libplatsupport: refactor more timers"
ef1dfb7da52b8324281ee6999951bf603d44bdd7

Revert "libplatsupport: riscv plic irqchip parser"

This reverts commit 23092562396ae9209679c83d75d25a953b70c2c6.

libplatsupport: riscv plic irqchip parser

Add irqchip parser for riscv plic.