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/P2020RDB-PC_36BIT_SDCARD_defconfig b/configs/P2020RDB-PC_36BIT_SDCARD_defconfig
index 4c524df..0c7da8f 100644
--- a/configs/P2020RDB-PC_36BIT_SDCARD_defconfig
+++ b/configs/P2020RDB-PC_36BIT_SDCARD_defconfig
@@ -50,6 +50,7 @@
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_DM=y
+CONFIG_DDR_CLK_FREQ=66666666
 CONFIG_DM_I2C=y
 CONFIG_SPL_SYS_I2C_LEGACY=y
 CONFIG_I2C_SET_DEFAULT_BUS_NUM=y