ARM: export default read_current_timer

ARM: export default read_current_timer

read_current_timer is used by get_cycles since "ARM: 7538/1: delay:
add registration mechanism for delay timer sources", and get_cycles
can be used by device drivers in loadable modules, so it has to
be exported.

Without this patch, building imote2_defconfig fails with

ERROR: "read_current_timer" [crypto/tcrypt.ko] undefined!

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Jonathan Austin <jonathan.austin@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>