configs: Convert AT91RESET_EXTRST to Kconfig

Convert AT91RESET_EXTRST to Kconfig for easier integration. The symbol is
not configurable from menuconfig, it will be automatically selected by SoCs
that select it explicitly via the "select" directive.
AT91RESET_EXTRST sets the Reset Controller's RSTC_CR.EXTRST bit which
asserts the nrst_out pin that resets external devices.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>