Convert CONFIG_AT91_EFLASH to Kconfig

This converts the following to Kconfig:
   CONFIG_AT91_EFLASH

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Acked-by: Eugen Hristev <eugen.hristev@microchip.com>