arm: dts: k3-am642-evm-u-boot: Add u-boot, dm-spl tag in the pinmux node of mmc1

Add u-boot,dm-spl tag in the pinmux device tree node, required for MMCSD1
subsystem.

Fixes: b6059ddc ("arm: dts: k3-am642: Add r5 specific dt support")
Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>