nios: Move README.nios_CONFIG_SYS_NIOS_CPU to doc/ dir

Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>
diff --git a/README.nios_CONFIG_SYS_NIOS_CPU b/doc/README.nios_CONFIG_SYS_NIOS_CPU
similarity index 100%
rename from README.nios_CONFIG_SYS_NIOS_CPU
rename to doc/README.nios_CONFIG_SYS_NIOS_CPU