zynq: limit SDHCI-driver build to selected boards

This patch introduces the spec value 'zync_sdhci' restrict the build of
the driver to the boards that actually support it.

Fixes #37

mk/spec/zynq_parallella.mk

@@ -1,7 +1,7 @@
 #
 # Pull in CPU specifics
 #
-SPECS += zynq cadence_gem 
+SPECS += zynq cadence_gem zynq_sdhci
 
 REP_INC_DIR += include/spec/parallella
 REP_INC_DIR += include/spec/xilinx

mk/spec/zynq_zc702.mk

@@ -1,7 +1,7 @@
 #
 # Pull in CPU specifics
 #
-SPECS += zynq cadence_gem zynq_uart1
+SPECS += zynq cadence_gem zynq_uart1 zynq_sdhci
 
 REP_INC_DIR += include/spec/zc702
 REP_INC_DIR += include/spec/xilinx

mk/spec/zynq_zc706.mk

@@ -1,7 +1,7 @@
 #
 # Pull in CPU specifics
 #
-SPECS += zynq cadence_gem zynq_uart1
+SPECS += zynq cadence_gem zynq_uart1 zynq_sdhci
 
 REP_INC_DIR += include/spec/zc706
 REP_INC_DIR += include/spec/xilinx

src/drivers/sd_card/spec/zynq/target.mk

@@ -1,5 +1,5 @@
 TARGET   = sd_card_drv
-REQUIRES = zynq
+REQUIRES = zynq_sdhci
 SRC_CC   = main.cc
 LIBS     = base server
 INC_DIR += $(PRG_DIR)