dt-bindings: mmc: sdhci-am654: Add compatible string for AM64 SoC

commit 41ed306ab2a13d95e2150027f2645f012bc9d15c from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git

Add compatible string for AM64 SoC in device tree binding of AM654 SDHCI
module as the same IP is used.

Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>