arm64: dts: specify 1.8V EMMC capabilities for bcm958742t

Specify 1.8V EMMC capabilities for bcm958742t board to indicate support
for UHS mode.

Fixes: d4b4aba6 ("arm64: dts: Initial DTS files for Broadcom Stingray SOC")
Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>