ARM: at91/dt: sama5d4: Fix mmc0 pinmux comments

Synchronise the comments in mmc0 pinmux node with HSMCI0 pinmux
description in the Atmel SAMA5D4 datasheet from 24-Aug-15 page
1119, section 37.6.1, Table 37-3 .

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Reviewed-by: Josh Wu <josh.wu@atmel.com>
Reported-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>