riscv: Eswin hwreset support added.

Add reset controller driver for Eswin EIC7700.

This one has two reset cells in FDT, which is unusual, so provide a
custom hwreset_map method.

Tested on SiFive P

riscv: Eswin hwreset support added.

Add reset controller driver for Eswin EIC7700.

This one has two reset cells in FDT, which is unusual, so provide a
custom hwreset_map method.

Tested on SiFive Premier P550.

Reviewed by: mhorne, jrtc27
Differential Revision: https://reviews.freebsd.org/D47853

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