power: reset: vexpress: fix build issue

power: reset: vexpress: fix build issue

An allmodconfig kernel makes CONFIG_VEXPRESS_CONFIG a module and
CONFIG_POWER_RESET_VEXPRESS builtin. That makes us see this build
error:

aarch64-linux-gnu-ld: drivers/power/reset/vexpress-poweroff.o: in function `vexpress_reset_probe':
../drivers/power/reset/vexpress-poweroff.c:119: undefined reference to `devm_regmap_init_vexpress_config'
../drivers/power/reset/vexpress-poweroff.c:119:(.text+0x48c): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol
`devm_regmap_init_vexpress_config'
make[1]: *** [/srv/src/kernel/next/Makefile:1126: vmlinux] Error 1

Rework so that POWER_RESET_VEXPRESS depends on 'VEXPRESS_CONFIG=y'.

Link: https://lore.kernel.org/r/20200527112608.3886105-5-anders.roxell@linaro.org
Fixes: d06cfe3f123c ("bus: vexpress-config: Merge vexpress-syscfg into vexpress-config")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>