ARM: rmobile: Fix eMMC signal voltage on Salvator-X/XS

The eMMC is 1V8 device only and the signaling is always 1V8,
fix the DT for Salvator-X/XS to describe the hardware correctly.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>