powerpc/embedded6xx: Fix no previous prototype for avr_uart_send() etc.

Move the prototypes into mpc10x.h which is included by all the relevant
C files, fixes:

  arch/powerpc/platforms/embedded6xx/ls_uart.c:59:6: error: no previous prototype for 'avr_uart_configure'
  arch/powerpc/platforms/embedded6xx/ls_uart.c:82:6: error: no previous prototype for 'avr_uart_send'

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240305123410.3306253-1-mpe@ellerman.id.au