[ARM] pxa: remove machine class specific stuffs from serial driver

The only things prevent drivers/serial/pxa.c from being generic enough
are:

  1. IER_UUE which can be safely replaced by UART_IER_UUE as defined in
     serial_reg.h for PXA

  2. __PREG() and FFUART/BTUART/STUART definitions to decide the UART
     port name

And removed the un-necessary #include of <mach/...> and <asm/...> headers.

Signed-off-by: Eric Miao <eric.miao@marvell.com>