phy: cadence: Sierra: Move all reset_control_get*() to a separate function

No functional change. Group devm_reset_control_get() and
devm_reset_control_get_optional() to a separate function.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>