arm64: Handle arbitrary CONFIG_SYS_MALLOC_F_LEN values

The encoding of the sub instruction used to handle CONFIG_SYS_MALLOC_F_LEN
can only accept certain values, and the set of acceptable values differs
between the AArch32 and AArch64 instructions sets. The default value of
CONFIG_SYS_MALLOC_F_LEN works with either ISA. Tegra uses a non-default
value that can only be encoded in the AArch32 ISA. Fix the AArch64 crt0
assembly so it can handle completely arbitrary values.

Signed-off-by: Thierry Reding <treding@nvidia.com>
[twarren: trimmed Thierry's patch to remove changes already present]
Signed-off-by: Tom Warren <twarren@nvidia.com>
[swarren, cleaned up patch, wrote description, re-wrote subject]
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/arm/lib/crt0_64.S

@@ -74,7 +74,8 @@ zero_gd:
 	cmp	x0, x18
 	b.gt	zero_gd
 #if defined(CONFIG_SYS_MALLOC_F_LEN)
-	sub	x0, x18, #CONFIG_SYS_MALLOC_F_LEN
+	ldr	x0, =CONFIG_SYS_MALLOC_F_LEN
+	sub	x0, x18, x0
 	str	x0, [x18, #GD_MALLOC_BASE]
 #endif
 	bic	sp, x0, #0xf	/* 16-byte alignment for ABI compliance */