microblaze: Compile uart 16550 serial driver

Upcomming little endian platform will use 16550 serial driver.
Signed-off-by: NMichal Simek <monstr@monstr.eu>
Signed-off-by: NEdgar E. Iglesias <edgar.iglesias@petalogix.com>

default-configs/microblaze-softmmu.mak

@@ -2,3 +2,4 @@
 
 CONFIG_PTIMER=y
 CONFIG_PFLASH_CFI01=y
+CONFIG_SERIAL=y

default-configs/microblazeel-softmmu.mak

@@ -2,3 +2,4 @@
 
 CONFIG_PTIMER=y
 CONFIG_PFLASH_CFI01=y
+CONFIG_SERIAL=y