boards/atmega1284p: Fix duplicated includes & typo

- avrdude.mk and serial.mk were included twice.
    - The former resulted in `make flash` wasting one flash cycle by flashing
      the same firmware twice
- Fixed typos (atmega328p instead of atmega1284p)

boards/atmega1284p/Makefile.include

@@ -2,8 +2,7 @@
 PORT_LINUX          ?= /dev/ttyUSB0
 PORT_DARWIN         ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
 BAUD                ?= 9600
-ATMEGA328P_CLOCK    ?=
-include $(RIOTMAKE)/tools/serial.inc.mk
+ATMEGA1284P_CLOCK   ?=
 
 # Allow overwriting programmer via env variables without affecting other boards
 PROGRAMMER_BOARD_ATMEGA1284P ?= dragon_isp
@@ -14,5 +13,4 @@ ifneq (,$(ATMEGA1284P_CLOCK))
   CFLAGS += -DCLOCK_CORECLOCK=$(ATMEGA1284P_CLOCK)
 endif
 
-include $(RIOTMAKE)/tools/avrdude.inc.mk
 include $(RIOTBOARD)/common/atmega/Makefile.include

boards/atmega1284p/doc.txt

@@ -11,7 +11,7 @@ it. (An ISP programmer will be needed to program it; or to program a bootloader
 to subsequently allow programming via UART.)
 
 ### MCU
-| MCU           | ATmega328p                             |
+| MCU           | ATmega1284p                            |
 |:------------- |:-------------------------------------- |
 | Family        | AVR/ATmega                             |
 | Vendor        | Microchip (previously Atmel)           |