ARMv8: LS1043A: change macro CONFIG_ARMV8_PSCI definition

NXP/Freescale uses macro CONFIG_ARMV8_PSCI to enable their private PSCI
implementation in PPA firmware, but this macro naming too generic, so this
patch replaces it with a specic one CONFIG_FSL_PPA_ARMV8_PSCI.
And this macro CONFIG_ARMV8_PSCI will be used for a generic PSCI for ARMv8
which will be added in following patchs.

Signed-off-by: Hongbo Zhang <hongbo.zhang@nxp.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: York Sun <york.sun@nxp.com>