eeprom: Add bus argument to eeprom_init()

Add bus argument to eeprom_init(), so that it can select
the I2C bus number on which the eeprom resides. Any negative
value of the $bus argument will preserve the old behavior.
This is in place so that old code does not randomly break.

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>
[trini: Wrap i2c_set_bus_num() call with CONFIG_SYS_I2C test]
Signed-off-by: Tom Rini <trini@konsulko.com>