hw/sd/omap_mmc: Split 'pseudo-reset' from 'power-on-reset'

hw/sd/omap_mmc: Split 'pseudo-reset' from 'power-on-reset'

DeviceClass::reset models a "cold power-on" reset which can
also be used to powercycle a device; but there is no "hot reset"
(a.k.a. soft-reset) method available.

The OMAP MMC Power-Up Control bit is not designed to powercycle
a card, but to disable it without powering it off (pseudo-reset):

  Multimedia Card (MMC/SD/SDIO) Interface [SPRU765A]

  MMC_CON[11] Power-Up Control (POW)
  This bit must be set to 1 before any valid transaction to either
  MMC/SD or SPI memory cards.
  When 1, the card is considered powered-up and the controller core
  is enabled.
  When 0, the card is considered powered-down (system dependent),
  and the controller core logic is in pseudo-reset state. This is,
  the MMC_STAT flags and the FIFO pointers are reset, any access to
  MMC_DATA[DATA] has no effect, a write into the MMC.CMD register
  is ignored, and a setting of MMC_SPI[STR] to 1 is ignored.

By splitting the 'pseudo-reset' code out of the 'power-on' reset
function, this patch fixes a latent bug in omap_mmc_write(MMC_CON)i
recently exposed by ecd219f7abb.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180706162155.8432-2-f4bug@amsat.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>