armv8: ls1088a: Add clock related function in CONFIG_SPL_BUILD

get_board_ddr_clk(), get_board_sys_clk() and if_board_diff_clk() is
now available for SPL build.

Signed-off-by: Ashish Kumar <Ashish.Kumar@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>