drivers: serial: nrf5: Undefine MDK macros to avoid conflict

The new Nordic MDK redefines certain UART signal names for backwards
compatibility with the new nRF52840 IC.
This patch addresses the potential name conflict between the redefined
signals and the existing variable names defined in the UART driver.

JIRA: ZEP-1418

Change-Id: I559d9335d2eeaa0a88d3f261b70508a1c98b989d
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>