armv8: ls1046ardb: update core frequency to 1800MHZ

Update the default core frequency to 1800MHZ for best performance under
SD boot and eMMC boot.
Signed-off-by: NGong Qianyu <Qianyu.Gong@nxp.com>
Reviewed-by: NYork Sun <york.sun@nxp.com>

board/freescale/ls1046ardb/ls1046ardb_rcw_emmc.cfg

 #PBL preamble and RCW header
 aa55aa55 01ee0100
 # RCW
-0c150010 0e000000 00000000 00000000
+0c150012 0e000000 00000000 00000000
 11335559 40000012 60040000 c1000000
 00000000 00000000 00000000 00238800
 20124000 00003000 00000096 00000001

board/freescale/ls1046ardb/ls1046ardb_rcw_sd.cfg

 #PBL preamble and RCW header
 aa55aa55 01ee0100
 # RCW
-0c150010 0e000000 00000000 00000000
+0c150012 0e000000 00000000 00000000
 11335559 40005012 60040000 c1000000
 00000000 00000000 00000000 00238800
 20124000 00003101 00000096 00000001