The Debian packaging of mbedtls is maintained in git, using the
merging workflow described in dgit-maint-merge(7).  There isn't a
patch queue that can be represented as a quilt series.

A detailed breakdown of the changes is available from their canonical
representation - git commits in the packaging repository.  For
example, to see the changes made by the Debian maintainer in the first
upload of upstream version 1.2.3, you could use:

    % git clone https://git.dgit.debian.org/mbedtls
    % cd mbedtls
    % git log --oneline 1.2.3..debian/1.2.3-1 -- . ':!debian'

(If you have dgit, use `dgit clone mbedtls`, rather than plain `git
clone`.)

A single combined diff, containing all the changes, follows.
--- mbedtls-2.14.1.orig/include/mbedtls/bn_mul.h
+++ mbedtls-2.14.1/include/mbedtls/bn_mul.h
@@ -51,12 +51,28 @@
     ( !defined(__ARMCC_VERSION) || __ARMCC_VERSION >= 6000000 )
 
 /*
+ * GCC < 5.0 treated the x86 ebx (which is used for the GOT) as a
+ * fixed reserved register when building as PIC, leading to errors
+ * like: bn_mul.h:46:13: error: PIC register clobbered by ‘ebx’ in ‘asm’
+ *
+ * This is fixed by an improved register allocator in GCC 5+. From the
+ * release notes:
+ * Register allocation improvements: Reuse of the PIC hard register,
+ * instead of using a fixed register, was implemented on x86/x86-64
+ * targets. This improves generated PIC code performance as more hard
+ * registers can be used.
+ */
+#if defined(__GNUC__) && __GNUC__ < 5 && defined(__PIC__)
+#define MULADDC_CANNOT_USE_EBX
+#endif
+
+/*
  * Disable use of the i386 assembly code below if option -O0, to disable all
  * compiler optimisations, is passed, detected with __OPTIMIZE__
  * This is done as the number of registers used in the assembly code doesn't
  * work with the -O0 option.
  */
-#if defined(__i386__) && defined(__OPTIMIZE__)
+#if defined(__i386__) && defined(__OPTIMIZE__) && !defined(MULADDC_CANNOT_USE_EBX)
 
 #define MULADDC_INIT                        \
     asm(                                    \
@@ -636,7 +652,8 @@
            "r6", "r7", "r8", "r9", "cc"         \
          );
 
-#elif defined (__ARM_FEATURE_DSP) && (__ARM_FEATURE_DSP == 1)
+#elif (__ARM_ARCH >= 6) && \
+    defined (__ARM_FEATURE_DSP) && (__ARM_FEATURE_DSP == 1)
 
 #define MULADDC_INIT                            \
     asm(
