doc: update README.arm64

This file apparently hasn't seen an update in a while, so just sync
it with reality.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>

doc/README.arm64

@@ -2,23 +2,31 @@ U-Boot for arm64
 
 Summary
 =======
-No hardware platform of arm64 is available now. The U-Boot is
-simulated on Foundation Model and Fast Model for ARMv8.
+The initial arm64 U-Boot port was developed before hardware was available,
+so the first supported platforms were the Foundation and Fast Model for ARMv8.
+These days U-Boot runs on a variety of 64-bit capable ARM hardware, from
+embedded development boards to servers.
 
 Notes
 =====
 
-1. Currenly, U-Boot run at the highest exception level processor
-   supported and jump to EL2 or optionally EL1 before enter OS.
+1. U-Boot can run at any exception level it is entered in, it is
+   recommened to enter it in EL3 if U-Boot takes some responsibilities of a
+   classical firmware (like initial hardware setup, CPU errata workarounds
+   or SMP bringup). U-Boot can be entered in EL2 when its main purpose is
+   that of a boot loader. It can drop to lower exception levels before
+   entering the OS.
 
 2. U-Boot for arm64 is compiled with AArch64-gcc. AArch64-gcc
    use rela relocation format, a tool(tools/relocate-rela) by Scott Wood
    is used to encode the initial addend of rela to u-boot.bin. After running,
    the U-Boot will be relocated to destination again.
 
-3. Fdt should be placed at a 2-megabyte boundary and within the first 512
-   megabytes from the start of the kernel image. So, fdt_high should be
-   defined specially.
+3. Earlier Linux kernel versions required the FDT to be placed at a
+   2 MB boundary and within the same 512 MB section as the kernel image,
+   resulting in fdt_high to be defined specially.
+   Since kernel version 4.2 Linux is more relaxed about the DT location, so it
+   can be placed anywhere in memory.
    Please reference linux/Documentation/arm64/booting.txt for detail.
 
 4. Spin-table is used to wake up secondary processors. One location
@@ -37,9 +45,8 @@ Notes
    aarch32 specific codes.
 
 
-
-Contributor
-===========
+Contributors
+============
    Tom Rini            <trini@ti.com>
    Scott Wood          <scottwood@freescale.com>
    York Sun            <yorksun@freescale.com>