riscv: Add a SYSCON driver for Andestech's PLMT

The platform-Level Machine Timer (PLMT) block
holds memory-mapped mtime register associated
with timer tick.

This driver implements the riscv_get_time() which
is required by the generic RISC-V timer driver.

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: Greentime Hu <greentime@andestech.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 511768b..ae8ff7b 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -118,6 +118,15 @@
 	  The Andes PLIC block holds memory-mapped claim and pending registers
 	  associated with software interrupt.
 
+config ANDES_PLMT
+	bool
+	depends on RISCV_MMODE
+	select REGMAP
+	select SYSCON
+	help
+	  The Andes PLMT block holds memory-mapped mtime register
+	  associated with timer tick.
+
 config RISCV_RDTIME
 	bool
 	default y if RISCV_SMODE