mmc: atmel-mci: Provide 2 new and correct 1 existing property description

mmc: atmel-mci: Provide 2 new and correct 1 existing property description

'sg_len' and 'dma_conf' are missing correct kerneldoc documentation and
'buf_phys_addr' was misspelled.  Correcting these issues fixes the
following W=1 kernel build issues:

 mmc/host/atmel-mci.c:352: warning: Function parameter or member 'sg_len' not described in 'atmel_mci'
 mmc/host/atmel-mci.c:352: warning: Function parameter or member 'buf_phys_addr' not described in 'atmel_mci'
 mmc/host/atmel-mci.c:352: warning: Function parameter or member 'dma_conf' not described in 'atmel_mci'

Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200701124702.908713-7-lee.jones@linaro.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>