Revert "ARM: meson: Add support for fastboot_set_reboot_flag()"

With the introduction of a generic reboot flag implemented in commit
a362ce21 ("fastboot: Implement generic fastboot_set_reboot_flag"), we
no longer need the custom PSCI implementation to handle the reboot reason.

This reverts commit 9a34dedf.

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>