riscv: alloc space exhausted

When trying to run qemu-riscv64_smode_defconfig with 32 harts booting
fails. The debug UART shows a message

    alloc space exhausted

32 is the current maximum number of harts for machine virt in QEMU 7.0.

Raise the default for SYS_MALLOC_F_LEN to 16 KiB.

Move the setting to /Kconfig where we define SYS_MALLOC_F_LEN for
other architectures too.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/Kconfig b/Kconfig
index 1127454..678317e 100644
--- a/Kconfig
+++ b/Kconfig
@@ -248,7 +248,7 @@
 	hex "Size of malloc() pool before relocation"
 	depends on SYS_MALLOC_F
 	default 0x1000 if AM33XX
-	default 0x4000 if SANDBOX
+	default 0x4000 if SANDBOX || RISCV
 	default 0x2000 if (ARCH_IMX8 || ARCH_IMX8M || ARCH_MX7 || \
 			   ARCH_MX7ULP || ARCH_MX6 || ARCH_MX5 || \
 			   ARCH_LS1012A || ARCH_LS1021A || ARCH_LS1043A || \