avr32: add missing include causing undefined pgtable_page_* references

This patch adds the linux/mm.h header file to the AVR32 arch pgalloc.c
implementation to fix the undefined reference to pgtable_page_ctor() and
pgtable_page_dtor().

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>