Fix a coding style issue in trusty.c

Fix a coding style issue in trusty.c

This patch fixes the following coding style error reported
by the checkpatch.pl script:

  Bad function definition - void el3_exit() should probably
  be void el3_exit(void)

There is another one but it's a false positive so there's no
point in fixing it:

  space prohibited after that '&' (ctx:WxW)
  +#define SMC_NR(entity, fn, fastcall, smc64) ((((fastcall) & 0x1) << 31) | \
                                                            ^
Change-Id: I34de0337c7216dabd16395879f13845a60ee6df0

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>