drm/amdgpu: Remove redundant I2C EEPROM address

Remove redundant EEPROM_I2C_MADDR_54H address, since we already have it
represented (ARCTURUS), and since we don't include the I2C device type
identifier in EEPROM memory addresses, i.e. that high up in the device
abstraction--we only use EEPROM memory addresses, as memory is continuously
represented by EEPROM device(s) on the I2C bus.

Add a comment describing what these memory addresses are, how they come
about and how they're usually extracted from the device address byte.

Cc: Candice Li <candice.li@amd.com>
Cc: Tao Zhou <tao.zhou1@amd.com>
Cc: Alex Deucher <Alexander.Deucher@amd.com>
Fixes: c9bdc6c3

 ("drm/amdgpu: Add EEPROM I2C address support for ip discovery")
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Alex Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>