Add readme for nxp arm64 platforms

Signed-off-by: Shengzhou Liu <Shengzhou.Liu@nxp.com>

packages/platforms/nxp/README.txt

@@ -0,0 +1,51 @@
+How to build ONL for NXP arm64 platforms
+----------------------------------------
+$ cd OpenNetworkLinux
+$ export VERSION=9
+$ make docker
+ONL installer will be generated in builds/arm64/installer/swi/builds directory.
+
+
+How to install ONL via ONIE
+----------------------------------------
+There are two ways to install ONL as below
+- Put ONL installer under the service directory of TFTP server
+- Put ONL installer on /dev/sda1, /dev/sda2 or /dev/sda3 under the root directory
+with name onie-installer-arm64.bin or onie-installer.bin
+e.g. /dev/sdb1/onie-installer-arm64-nxp_ls1046ardb.bin
+ONIE will automatically search the installer from /dev/sda(1/2/3) or download it
+from TFTP server, then run the installer to install ONL to target storage drive.
+
+
+NXP arm64 platforms supported in ONL
+------------------------------------
+LS1043ARDB  (install on /dev/mmcblk0, SD/MMC card)
+LS1046ARDB  (install on /dev/mmcblk0, SD/MMC card)
+LS1088ARDB  (install on /dev/mmcblk0, SD/MMC card)
+LS2088ARDB  (install on /dev/sda,     USB stick)
+
+
+ONL Linux LTS-4.9 features verified on NXP arm64 platforms
+----------------------------------------------------------
+-Linux SMP
+-Linux reboot
+-GIC
+-Generic Timer
+-SMMU
+-Flextimer
+-IFC NOR
+-IFC NAND
+-DSPI
+-QSPI
+-UART
+-I2C
+-I2C RTC
+-USB
+-PCIe
+-SATA
+-SDHC
+-MC bus
+-MC console
+-VFIO
+-DPAA Ethernet
+-DPAA2 Ethernet