mmc: sdhci-spear: remove support for power gpio

None of this code is currently used: there are no definitions of
struct sdhci_plat_data in arch/arm, neither are there any DT properties
which use card_power_gpio/power_active_high/power_always_enb.  In any
case, slot power control should be rigged up via vmmc and the regulator
subsystem in the DT case.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Chris Ball <chris@printf.net>