cpu/arm/k60: UART: small fixes

 - Inline IRQ handlers
 - Add volatile keyword to all UART_Type pointers.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: proper function declaration of get_next_dev_fd.

Internal helper function in newlib-syscalls.c was missing a declaration
before the definition.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
Fix too short 6LoWPAN reassembly timeout
cpu/arm/k60: add -fshort-enums to CFLAGS.

Added -fshort-enums for compatibility with newlib built by crossdev.
Needs testing on Ubuntu etc. to verify that -fshort-enum is the
default on those packages as well.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>