Initial merge of Nordic Semi nRF51/52 from Unicore MX back into Libopencm3

* merged: nrf tree from unicore-mx
* fixed: small changes to make merged code play with rest of locm3 again
* added: linker script generator defines for nRF51/52 stubs
* added: doxygen support

This removes code and changes names and styles where relevant to be more
inline with normal libopencm3.

NRF52x library is built for hardfloat, M4F by default.  The M4 no float
variants are less common, and if needed, the library can be built
manually for those variants.  Unless some very common boards show up
using those parts, we don't need an extra library build.

Reviewed-by: Karl Palsson <karlp@tweak.net.au>
Tested-by: Karl Palsson <karlp@tweak.net.au>

lib/nrf/52/Makefile

@@ -0,0 +1,50 @@
+##
+## This file is part of the libopencm3 project.
+##
+## Copyright (C) 2017-2018 Unicore MX project<dev(at)lists(dot)unicore-mx(dot)org>
+## Copyright (C) 2021 Eduard Drusa <ventyl86(at)netkosice(dot)sk>
+##
+## This library is free software: you can redistribute it and/or modify
+## it under the terms of the GNU Lesser General Public License as published by
+## the Free Software Foundation, either version 3 of the License, or
+## (at your option) any later version.
+##
+## This library is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+## GNU Lesser General Public License for more details.
+##
+## You should have received a copy of the GNU Lesser General Public License
+## along with this library.  If not, see <http://www.gnu.org/licenses/>.
+##
+
+LIBNAME		= libopencm3_nrf52
+SRCLIBDIR	?= ../..
+
+CC		= $(PREFIX)gcc
+AR		= $(PREFIX)ar
+
+FP_FLAGS ?= -mfloat-abi=hard -mfpu=fpv4-sp-d16
+
+TGT_CFLAGS	= -Os -Wall -Wextra -I../../../include -fno-common \
+		  -mcpu=cortex-m4 -mthumb $(FP_FLAGS) \
+		  -Wredundant-decls -Wmissing-prototypes -Wstrict-prototypes \
+		  -ffunction-sections -fdata-sections -MD -DNRF52
+TGT_CFLAGS += $(DEBUG_FLAGS)
+TGT_CFLAGS += $(STANDARD_FLAGS)
+# ARFLAGS	= rcsv
+ARFLAGS		= rcs
+ 
+OBJS += clock_common.o
+OBJS += gpio.o
+OBJS += i2c.o
+OBJS += ppi.o
+OBJS += radio_common.o
+OBJS += rtc.o
+OBJS += timer.o
+OBJS += uart.o
+
+VPATH += ../../cm3:../common
+
+include ../../Makefile.include
+