arm: dts: k3-am65: Fix mmc nodes

Because of fundamental interface issues in am65x pg1, only the
initial sdhci1 node at 25 MHz was added in the u-boot.dtsi
from which both the base-board.dts and r5-base-board.dts
inherit the node. Move the node out to k3-am65-main.dtsi
where it belongs and add the board specific properties
in base-board.dts and r5-base-board.dts

This ensures dts compatibility with the kernel dts in the
base-board.dts and enables the SD card interface at 50 MHz
and High Speed mode

While we are here, also fix the main_mmc0_pins_default
property to be included and inherit from the base-board.dts
instead of the u-boot.dtsi

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>