Cunman,
MMCSD_CLK pin provides the clock to the memory card from the MMC/SD controller.
MMC/SD Clocking:
Synchronous peripherals have their frequencies derived from the clock frequency.
The peripheral system clock frequency changes accordingly, if the PLL0 frequency changes.
Most synchronous peripherals have internal dividers so they can generate their required clock frequencies.
For more details refer the section "Clock Control" at the OMAP-L138 TRM(spruh77).