lsm303d: added 'lsm303d regdump' command

useful for diagnosing issues
lsm303d: define some more register addresses
lsm303d: print more registers in "lsm303d regdump"
SPI: added set_frequency() API

this allows the bus speed to be changed on the fly by device
drivers. This is needed for the MPU6000
mpu6000: change bus speed based on registers being accessed

this ensures we follow the datasheet requirement of 1MHz for general
registers and up to 20MHz for sensor and int status registers