nvmem: layouts: sl28vpd: set varaiable sl28vpd_layout storage-class-specifier to static

smatch reports
drivers/nvmem/layouts/sl28vpd.c:144:21: warning: symbol
  'sl28vpd_layout' was not declared. Should it be static?

This variable is only used in one file so it should be static.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>