soc: xilinx: zynq7000: move to using DT_STRING_UPPER_TOKEN_BY_IDX macro

Replaced hand coded use with DT_STRING_UPPER_TOKEN_BY_IDX.  We should have
introduced the APIs when this was needed since the token values in
devicetree_generated.h should only be accessed via an API macro.

Signed-off-by: Kumar Gala <galak@kernel.org>