xtensa: fix DTC warning unit_address_format

DTC issues the following warnings when building xtfpga device trees:

 /soc/flash@00000000/partition@0x0: unit name should not have leading "0x"
 /soc/flash@00000000/partition@0x6000000: unit name should not have leading "0x"
 /soc/flash@00000000/partition@0x6800000: unit name should not have leading "0x"
 /soc/flash@00000000/partition@0x7fe0000: unit name should not have leading "0x"

Drop leading 0x from flash partition unit names.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>