ARM: nommu: don't limit TASK_SIZE

With TASK_SIZE set to the maximal RAM address booting in some XIP
configurations fails (e.g. on efm32 DK3750). The problem is that
strncpy_from_user et al. check for the address not being above TASK_SIZE
(since 8c56cc8b (ARM: 7449/1: use generic strnlen_user and
strncpy_from_user functions)) and this makes booting fail if the XIP
flash is above the RAM address space.

This change is in line with blackfin, frv and m68k which also use
0xffffffff for TASK_SIZE with !MMU.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>