tiny-printf does not know about the "l" modifier so far, which breaks
the crash dump on AArch64, because it uses %lx to print the registers.
Add an easy way of handling longs correctly.

Using a relatively decent compiler (GCC 5.3.0) this does _not_ increase
the code size of tiny-printf.o for 32-bit builds (where long and int
are actually the same), actually it looses three (ARM Thumb2) instructions
from the actual SPL (numbers for orangepi_plus_defconfig):
  text    data     bss     dec     hex filename
   758       0       0     758     2f6 spl/lib/tiny-printf.o	before
 18839     488     232   19559    4c67 spl/u-boot-spl		before
   758       0       0     758     2f6 spl/lib/tiny-printf.o	after
 18833     488     232   19553    4c61 spl/u-boot-spl		after

This adds some substantial amount of code to a 64-bit build, though:
(taken after a later commit, which enables the ARM64 SPL build for sunxi)
  text    data     bss     dec     hex filename
  1542       0       0    1542     606 spl/lib/tiny-printf.o	before
 25830     392     360   26582    67d6 spl/u-boot-spl		before
  1758       0       0    1758     6de spl/lib/tiny-printf.o	after
 26040     392     360   26792    68a8 spl/u-boot-spl		after

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>