powerpc/microwatt: Select COMMON_CLK in order to get the clock framework

This is to allow us to select Litex MMC host controller driver, which
drives the litesdcard gateware.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Acked-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/Z5xtT47QsnFpxc5E@thinks.paulus.ozlabs.org

arch/powerpc/platforms/microwatt/Kconfig

@@ -6,6 +6,7 @@ config PPC_MICROWATT
 	select PPC_ICS_NATIVE
 	select PPC_ICP_NATIVE
 	select PPC_UDBG_16550
+	select COMMON_CLK
 	help
           This option enables support for FPGA-based Microwatt implementations.