With CONFIG_NVMEM=m and CONFIG_PHY_QCOM_QUSB2=y we get a link error from
calls to devm_nvmem_cell_get and nvmem_cell_read:

drivers/built-in.o: In function `qusb2_phy_probe':
binder.c:(.text+0x4750): undefined reference to `devm_nvmem_cell_get'
drivers/built-in.o: In function `qusb2_phy_init':
binder.c:(.text+0x489c): undefined reference to `nvmem_cell_read'

Fix this by adding a Kconfig dependency to ensure we can only have this
driver built in when the nvmem functions are also built in or we see the
empty stub functions. We can still build this driver as a module when the
nvmem core is build as module, too.

Fixes: deffad633413 ("phy: qcom-qusb2: New driver for QUSB2 PHY on Qcom chips")
Signed-off-by: Tobias Regnery <tobias.regnery@gmail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>