mmc: zynq_sdhci: Add weak function prototype

mmc: zynq_sdhci: Add weak function prototype

zynqmp_pm_is_function_supported() which checks feature support on som,
which is implemented in firmware_zynqmp.c driver. As mini configuration
does not use firmware driver, so create a weak function to avoid
compilation error on zynqmp mini configuration.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Acked-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/c60655a509956b8fc3a81671a7dc51157f3973db.1651048030.git.michal.simek@xilinx.com