[astro-thermal] Use IRQ instead of polling for trip points

ZX-2569 #done

Test: Ran Thermal Daemon on Astro

Change-Id: I97d6407d6f2daff5be3546d24c3b9fa83138194f

[astro-thermal] Set Initial voltage to support Max CPU freq.

Upon boot up we were setting the voltage which would be
sufficient for 1GHz CPU freq. But when zedbooting the CPU
freq. is already set to 1.893GHz and we lower the voltage during
the init. This was causing a stall. Setting the init voltage to
support 1.893GHz fixes the bug.

ZX-2559 #done

Test: Netboot Zircon on Astro.

Change-Id: Ia7f9b06614615c39da6c5d542143809ddf2df3e3

[astro-thermal] Astro CPU freq. set to MAX 1.893GHz

- Added initial hooks for thermal notifications.
- Using CPU freq. scaling to setup boot up freq. to be
1.893 GHz.
- A couple of small bugs in Voltage regulator code.

ZX-2498 #done
ZX-2507 #done

Test: Validated CPU freq. using clkctl utility.

Change-Id: Ib38372e90c19efe2cefdf78627f024809668a0fb

[astro-thermal] Add CPU Thermal policy

- Get thermal config & opps metadata, pass it to voltage
regulator.
- Add initial thermal policy for CPU.

Test: Boot up on Astro, no functional changes needed to be tested.

Change-Id: I927b3eafb8b234e5cd0a627fac3f367283b8acf2

[astro][voltage] Add Voltage Scaling support

Added interface to get/set voltage using the PWM
This will be later used for DVFS

Test: boot up on Astro
Change-Id: I9fc6b70af399b0ac9d0bb90034f0bfd1b94c5d5d