micropython: fix build for MIPS64 n32

This patch applies an upstream patch to fix a compile error like this
one:

modffi.c: In function 'ffifunc_call':
modffi.c:358:25: error: cast from pointer to integer of different size
[-Werror=pointer-to-int-cast]
             values[i] = (ffi_arg)a;

This error can be highlighted when building micropython for MIPS64 n32
because ffi_arg is 64-bit wide and the pointers on MIPS64 n32 are 32-bit
wide, so it's trying to case an integer to a pointer (or vice versa) of
a different size. We should cast first the pointer (or the integer) to a
pointer sized integer (intptr_t) to fix that problem.

This patch was merged upstream as a result of this pull request:

  https://github.com/micropython/micropython/pull/1471

Fixes:

  http://autobuild.buildroot.net/results/e22/e2253de3f96e9a53e75b4cecaf56c1df2950803f/



[Thomas: use a single assignement for MICROPYTHON_PATCH.]

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>