riscv: fix build break after macro-to-function conversion in generic cacheflush.h

Commit c296d4dc ("asm-generic: fix a compilation warning")
converted the various flush_*cache_* macros in
asm-generic/cacheflush.h to static inline functions.  This breaks
RISC-V builds, since RISC-V's cacheflush.h includes the generic
cacheflush.h and then undefines the macros to be overridden.

Fix by copying the subset of the no-op functions that are reused from
the generic cacheflush.h into the RISC-V cacheflush.h, and dropping
the include of the generic cacheflush.h.

Fixes: c296d4dc ("asm-generic: fix a compilation warning")
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Qian Cai <cai@lca.pw>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>