riscv: mark some code and data as file-static

Several functions and arrays which are only used in the files in which
they are declared are missing "static" qualifiers.  Warnings for these
symbols are reported by sparse:

arch/riscv/kernel/vdso.c:28:18: warning: symbol 'vdso_data' was not declared. Should it be static?
arch/riscv/mm/sifive_l2_cache.c:145:12: warning: symbol 'sifive_l2_init' was not declared. Should it be static?

Resolve these warnings by marking them as static.

This version incorporates feedback from Greentime Hu
<greentime.hu@sifive.com>.

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Greentime Hu <greentime.hu@sifive.com>