modules/allwinner: add nvmem_if.h to SRCS

Ensure the header is generated; it is a dependency for these drivers.
This fixes standalone module builds and riscv LINT.

Reported by: zlei
Sponsored by: The FreeBSD Foundation
Fixes: 5522519731b7 ("modules: enable allwinner kmods on riscv")

(cherry picked from commit fe7954d723f85abd6eaf035db92a3a60a7803eb4)