arm: dts: dra76x: Fix mmc3 max-frequency

dra76x is not affected by i887 which requires mmc3 node to be limited to
a max frequency of 64 MHz. Fix this by overwriting the correct value in
the the dra76 specific dtsi.

Fixes: 895bd4b3 ("ARM: dts: Add support for dra76-evm")
Cc: stable@vger.kernel.org
Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>