bhyve: Implement a PL031 RTC on arm64

Unlike amd64's, this RTC is implemented entirely in userspace. This is
the same RTC as is provided by QEMU's virt machine.

Reviewed by: jhb
MFC after: 2 weeks
Obtained from: CheriBSD

bhyve: Extract uart-clock from fdt_add_uart as an apb-pclk

This clock will also be used by the PL031 RTC (rather than defining
redundant per-device clocks).

Reviewed by: jhb
MFC after: 2 weeks
Obtained from: CheriBSD

bhyve: Support legacy PCI interrupts on arm64

This allows us to remove various #ifdef hacks and enable building more
PCI devices.

Note that a hole is left in the interrupt mapping for the RTC rather
than having the two core devices straddle the PCIe interrupts. QEMU's
virt machine also takes this approach.

Reviewed by: jhb
MFC after: 2 weeks
Obtained from: CheriBSD

bhyve: Add FDT building code for arm64

fdt.c provides some basic routines which let platform initialization
code build the FDT that gets passed into the guest. For now this is not
very generic; we declare info about CPUs, memory, a single UART
(specified by -o console), a PCIe controller (used for virtio devices),
an interrupt controller and the platform timer.

Co-authored-by: andrew
Reviewed by: corvink, jhb
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40996