nxp: Migrate CONFIG_DDR_CLK_FREQ to Kconfig

As this symbol can either be a fixed value or the function
get_board_ddr_clk, migration is tricky.  Introduce a choice of DYNAMIC
or STATIC_DDR_CLK_FREQ.  If DYNAMIC, we continue to use the board
defined get_board_ddr_clk function.  If STATIC, set CONFIG_DDR_CLK_FREQ
to that value and now include/clock_legacy.h contains the function
prototype or defines get_board_ddr_clk() to that static value.  Update
callers to test for DYNAMIC or STATIC.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/board/freescale/ls1021aqds/ls1021aqds.c b/board/freescale/ls1021aqds/ls1021aqds.c
index 711d8c2..fbbd27d 100644
--- a/board/freescale/ls1021aqds/ls1021aqds.c
+++ b/board/freescale/ls1021aqds/ls1021aqds.c
@@ -127,6 +127,7 @@
 	return 66666666;
 }
 
+#ifdef CONFIG_DYNAMIC_DDR_CLK_FREQ
 unsigned long get_board_ddr_clk(void)
 {
 	u8 ddrclk_conf = QIXIS_READ(brdcfg[1]);
@@ -141,6 +142,7 @@
 	}
 	return 66666666;
 }
+#endif
 
 int dram_init(void)
 {