mmc: atmel-mci: add support for odd clock dividers

Add an odd clock divider capability available from v5xx. It also involves
changing the clock divider calculation, and changing the switch-case
statement to use top-down fallthrough.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>