mx25l: Add support for Integrated Silicon Solution is25wp256

This is used for the on-board flash on the HiFive Unmatched board.

Reviewed by: riscv, jrtc27
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31562

(cherry picked from commit 416ac155bb750fa55917daf340abe4ef04e7d4e6)