arm: prevent using movt/movw address loads

The movt/movw instruction can be used to hardcode an
memory location in the instruction itself. The linker
starts complaining about this if the compiler decides
to do so: "relocation R_ARM_MOVW_ABS_NC against `a local
symbol' can not be used" and it is not support by U-boot
as well. Prevent their use by requiring word relocations.
This allows u-boot to be build at other optimalization
levels then -Os.

Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Cc: TigerLiu@viatech.com.cn
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>