at91rm9200: fix errors with CONFIG_CMD_I2C_TREE

This patch prevents linker error on AT91RM9200 boards, if
CONFIG_CMD_I2_TREE is set.
It implements i2c_set_bus_speed and i2c_get_bus_speed as a dummy function.

Signed-off-by: Jens Scharsig <esw@bus-elektronik.de>