ARM: shmobile: Include at24.c in shmobile_defconfig

The Koelsch board includes an at24-compatible EEPROM hooked up via I2C.
To increase hardware support enable the driver in the shmobile_defconfig
multiplatform configuration.

Signed-off-by: Magnus Damm <damm@opensource.se>
Acked-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>