RISC-V: Add Microchip PolarFire SoC kconfig option

Add Microchip PolarFire kconfig option which selects SoC specific
and common drivers that is required for this SoC.
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/Kconfig.socs

 menu "SoC selection"
 
+config SOC_MICROCHIP_POLARFIRE
+	bool "Microchip PolarFire SoCs"
+	select MCHP_CLK_MPFS
+	select SIFIVE_PLIC
+	help
+	  This enables support for Microchip PolarFire SoC platforms.
+
 config SOC_SIFIVE
 	bool "SiFive SoCs"
 	select SERIAL_SIFIVE if TTY