ARM: lpc32xx: allow multiplatform build

All preparation work is done, so the platform can finally
be moved into ARCH_MULTIPLATFORM. This requires a small
change to the defconfig file to enable the platform.

Link: https://lore.kernel.org/r/20190809144043.476786-14-arnd@arndb.de


Signed-off-by: Arnd Bergmann <arnd@arndb.de>