Make this function weak and implement it's weak implementation
so that the boards can just reimplement it. This zaps the horrid
CONFIG_SYS_EEPROM_WREN macro.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Cc: Heiko Schocher <hs@denx.de>
Reviewed-by: Heiko Schocher <hs@denx.de>