feat(plat/zynqmp): fix section `coherent_ram' will not fit in region `RAM'

Actually BL31_LIMIT is set to 0xffffffff but that doesn't work correctly
with bl31.ld since ". = ALIGN(((1) << (12)));" will try to fill aligned up
to 0x100000000 included, but the RAM size is 0xffffffff, so this leads to
this build error:
```
bl31.elf section `coherent_ram' will not fit in region `RAM'
/home/br-user/git/upstream/ci-tests/zynqmp_zcu102/host/bin/aarch64-buildroot-linux-uclibc-ld: region `RAM' overflowed by 1 byte
```
So let's move BR31_LIMIT to 0x100000000 giving 1 byte more room to fill RAM
up to the end.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Change-Id: Ic0edb8ed159e013f60598a9dd4f50adbf656b38d

plat/xilinx/zynqmp/include/platform_def.h

@@ -38,7 +38,7 @@
 #ifndef ZYNQMP_ATF_MEM_BASE
 #if !DEBUG && defined(SPD_none) && !SDEI_SUPPORT
 # define BL31_BASE			0xfffea000
-# define BL31_LIMIT			0xffffffff
+# define BL31_LIMIT			0x100000000
 #else
 # define BL31_BASE			0x1000
 # define BL31_LIMIT			0x7ffff