sdhci: fix CAPAB/MAXCURR registers, both are 64bit and read-only

running qtests:

  $ make check-qtest-arm
    GTESTER check-qtest-arm
  SDHC rd_4b @0x44 not implemented
  SDHC wr_4b @0x40 <- 0x89abcdef not implemented
  SDHC wr_4b @0x44 <- 0x01234567 not implemented

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 20180115182436.2066-12-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
