phy: qcom-qusb2: add NVMEM dependency

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>

drivers/phy/Kconfig

@@ -450,6 +450,7 @@ config PHY_QCOM_QMP
 config PHY_QCOM_QUSB2
 	tristate "Qualcomm QUSB2 PHY Driver"
 	depends on OF && (ARCH_QCOM || COMPILE_TEST)
+	depends on NVMEM || !NVMEM
 	select GENERIC_PHY
 	help
 	  Enable this to support the HighSpeed QUSB2 PHY transceiver for USB