regmap: select REGMAP if REGMAP_MMIO and REGMAP_IRQ enabled

The regmap_mmio and regmap_irq depend on regmap core, if not select,
we may not compile regmap core and meet compiling errors as follows
if REGMAP_MMIO is selected by client drivers:
drivers/mfd/syscon.c:94:15: error: variable 'syscon_regmap_config' has initializer but incomplete type
drivers/mfd/syscon.c:95:2: error: unknown field 'reg_bits' specified in initializer
drivers/mfd/syscon.c:95:2: warning: excess elements in struct initializer [enabled by default]
drivers/mfd/syscon.c:95:2: warning: (near initialization for 'syscon_regmap_config') [enabled by default]
drivers/mfd/syscon.c:96:2: error: unknown field 'val_bits' specified in initializer
drivers/mfd/syscon.c:96:2: warning: excess elements in struct initializer [enabled by default]
drivers/mfd/syscon.c:96:2: warning: (near initialization for 'syscon_regmap_config') [enabled by default]
drivers/mfd/syscon.c:97:2: error: unknown field 'reg_stride' specified in initializer
drivers/mfd/syscon.c:97:2: warning: excess elements in struct initializer [enabled by default]
drivers/mfd/syscon.c:97:2: warning: (near initialization for 'syscon_regmap_config') [enabled by default]
drivers/mfd/syscon.c: In function 'syscon_probe':
drivers/mfd/syscon.c:124:2: error: invalid use of undefined type 'struct regmap_config'
drivers/mfd/syscon.c:125:2: error: implicit declaration of function 'devm_regmap_init_mmio' [-Werror=implicit-function-declaration]
drivers/mfd/syscon.c:125:17: warning: assignment makes pointer from integer without a cast [enabled by default]
cc1: some warnings being treated as errors

drivers/mfd/Kconfig:
config MFD_SYSCON
        bool "System Controller Register R/W Based on Regmap"
	depends on OF
	select REGMAP_MMIO
	help
	  Select this option to enable accessing system control registers
	    via regmap.

Signed-off-by: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>