hw/ppc/Kconfig: Add missing dependency E500 -> DS1338 RTC

Commit 7abb479c ("PPC: E500: Add FSL I2C controller and integrate
RTC with it") added a global dependency on the DS1338 model, instead
of a machine one (via Kconfig). This gives trouble when building
standalone machines not exposing I2C bus:

  The following clauses were found for DS1338

      CONFIG_DS1338=y
      config DS1338 depends on I2C

Fix by selecting the DS1338 symbol in the single machine requiring
it, the E500.

Fixes: 7abb479c ("PPC: E500: Add FSL I2C controller and integrate RTC with it")
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210513163858.3928976-9-philmd@redhat.com>

configs/devices/ppc-softmmu/default.mak

 # Default configuration for ppc-softmmu
 
 # For embedded PPCs:
-CONFIG_DS1338=y
 CONFIG_E500=y
 CONFIG_PPC405=y
 CONFIG_PPC440=y

hw/ppc/Kconfig

@@ -130,6 +130,7 @@ config E500
     select SERIAL
     select MPC_I2C
     select FDT_PPC
+    select DS1338
 
 config VIRTEX
     bool