can: flexcan: remove HAVE_CAN_FLEXCAN Kconfig symbol

This patch removes the Kconfig symbol HAVE_CAN_FLEXCAN from arch/{arm,powerpc}
and allowing compilation unconditionally on all arm and powerpc platforms.

This brings a bigger compile time coverage and removes the following dependency
warning found by Arnd Bergmann:

    warning: (SOC_IMX28 && SOC_IMX25 && SOC_IMX35 && IMX_HAVE_PLATFORM_FLEXCAN &&
        SOC_IMX53 && SOC_IMX6Q) selects HAVE_CAN_FLEXCAN
    which has unmet direct dependencies (NET && CAN && CAN_DEV)

Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>