Convert socfpga: select CONFIG_HW_WATCHDOG support for ARCH_SOCFPGA

All Socfpga boards from ./include/configs/socfpga_* define
CONFIG_HW_WATCHDOG.
To ease CONFIG_HW_WATCHDOG conversion to Kconfig select it in
config ARCH_SOCFPGA (arch/arm/Kconfig) section.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>