"init_data", when non-NULL, is on the heap, and the register_chipv6_phy call
sometimes modifies data in (at least) the offset range [128:249], suggesting
that it is a buffer larger than 128 bytes in size (the size of our
"phy_init_data" buffer). When we use our static buffer (prior to this
change), the call could would overwrite the .rodata section and lead to
undefined behaviour.
To address this, just patch the heap-allocated buffer with our data.
Move phy_init_data to flash as it's now readonly and never modified.
