[riscv] Support mapping I/O devices outside of the identity map

With the 64-bit paging schemes (Sv39, Sv48, and Sv57), we identity-map
as much of the physical address space as is possible.  Experimentation
shows that this is not sufficient to provide access to all I/O
devices.  For example: the Sipeed Lichee Pi 4A includes a CPU that
supports only Sv39, but places I/O devices at the top of a 40-bit
address space.

Add support for creating I/O page table entries on demand to map I/O
devices, based on the existing design used for x86_64 BIOS.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/config/defaults/sbi.h

@@ -10,12 +10,17 @@
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #define IOAPI_RISCV
-#define IOMAP_VIRT
 #define DMAAPI_FLAT
 #define UACCESS_OFFSET
 #define TIMER_ZICNTR
 #define ENTROPY_ZKR
 
+#if __riscv_xlen == 64
+#define IOMAP_SVPAGE
+#else
+#define IOMAP_VIRT
+#endif
+
 #define CONSOLE_SBI
 #define REBOOT_SBI
 #define UMALLOC_UHEAP