arm: dts: an7581: set r_smpl for MMC in U-Boot

arm: dts: an7581: set r_smpl for MMC in U-Boot

When booting from SPI, the ROM code does not initialize the MMC
controller on AN7581. As a result, the first MMC initialization
is performed by U-Boot.

In this case, the r_smpl bit is left uninitialized, which may
cause incorrect sampling timing during early MMC access.

Set the r_smpl bit explicitly in the U-Boot device tree to ensure
reliable MMC initialization.

This change is limited to the U-Boot-specific device tree.
The Linux MMC driver already performs runtime delay detection
and does not require a fixed r_smpl setting.

Signed-off-by: Ray Liu <ray.xy.liu@gmail.com>