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/configs/ls1046aqds_defconfig b/configs/ls1046aqds_defconfig
index 72c1728..1649db4 100644
--- a/configs/ls1046aqds_defconfig
+++ b/configs/ls1046aqds_defconfig
@@ -46,6 +46,7 @@
 CONFIG_DM=y
 CONFIG_SATA_CEVA=y
 CONFIG_FSL_CAAM=y
+CONFIG_DYNAMIC_DDR_CLK_FREQ=y
 CONFIG_DM_I2C=y
 CONFIG_I2C_SET_DEFAULT_BUS_NUM=y
 CONFIG_SYS_I2C_EEPROM_ADDR=0x57