[ARM] pxa: refactor uncompress.h for non-PXA uarts

The original patch came from Marc Zyngier where support of 8250-compatible
UART is required to show the uncompress information. Modified a little bit
here, including changes below:

1. #include <mach/regs-uart.h> is actually not necessary
2. introduced uart_{read,write}() for different base and shift
3. introduced uart_is_enabled() and assumed enabled always for
   non-PXA uarts

Signed-off-by: Eric Miao <eric.y.miao@gmail.com>
Acked-by: Marc Zyngier <maz@misterjones.org>