of: add of_property_read_variable_* dummy helpers

Commit a67e9472 ("of: Add array read functions with min/max size
limits") added a new interface for reading variable-length arrays from
DT properties. One user was added in dsa recently and this causes a
build error because that code can be built with CONFIG_OF disabled:

net/dsa/dsa2.c: In function 'dsa_switch_parse_member_of':
net/dsa/dsa2.c:678:7: error: implicit declaration of function 'of_property_read_variable_u32_array'; did you mean 'of_property_read_u32_array'? [-Werror=implicit-function-declaration]

This adds a dummy functions for of_property_read_variable_u32_array()
and a few others that had been missing here. I decided to move
of_property_read_string() and of_property_read_string_helper() in the
process to make it easier to compare the two sets of function prototypes
to make sure they match.

Fixes: 975e6e32

 ("net: dsa: rework switch parsing")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>