of: remove special case definition of of_read_ulong()

Special case of of_read_ulong() was defined for PPC32 to toss away
all but the last 32 bits when a large number value was read, and the
'normal' version for ppc64 just #defined of_read_ulong to of_read_number
which causes compiler warnings on MicroBlaze and other 32 bit
architectures because it returns a u64 instead of a ulong.

This patch fixes the problem by defining a common implementation of
of_read_ulong() that works everywhere.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>
Tested-by: Michal Simek <monstr@monstr.eu>