ipq806x: 5.15: drop mmc-ddr-1_8v from sdcc1 node

ipq806x: 5.15: drop mmc-ddr-1_8v from sdcc1 node

Zyxel NGB6817 is the only router that use mmc for rootfs. Upstream
kernel dtsi have mmc-ddr-1_8v enabled for sddc1. This is wrong as mmc on
ipq806x is supplied by a fixed 3.3v regulator and can't operate at 1.8v.
This cause the sddc1 to malfunction and cause kernel panic.
In old 5.15 version this was disabled but it was put in addition to many
other changes so it was dropped silently. Restore this patch to fix
working condition of such router.

Fixes: 88bf652 ("ipq806x: 5.15: replace dtsi patches with upstream version")
Fixes: #11000
Tested-by: Hendrik Koerner <koerhen@web.de>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>