riscv: support the new "riscv,isa-extensions" string-array.HEADmain

Support the new "riscv,isa-extensions" property on RISC-V hart nodes
in FDT.

The "riscv,isa" property is deprecated, but cannot be removed because
doing so would break compatibility with existing DTBs. The new properties
replace it: "riscv,isa-base" describes the base ISA and
"riscv,isa-extensions" is a string array containing the supported ISA
extensions.

The "riscv,isa-extensions" property can be relatively large; on the
Spacemit K3 SoC it is approximately 300 bytes.

The FreeBSD OFW interface does not provide access to the underlying FDT
property data without copying it, and memory allocation is not possible
this early. So allocate a static buffer for the property instead.

Reuse the existing parse_riscv_isa() implementation to parse both the new
extension property and the legacy "riscv,isa" property.

Reviewed by:	bnovkov, mhorne
Sponsored by:	EPSRC ("UKRI3001: CHERI Research Centre")
Differential Revision:	https://reviews.freebsd.org/D60209

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