arch: riscv: Allow skipping default ROM PMP entry

The `z_riscv_pmp_init` function currently includes a standard PMP entry
to mark the ROM region (`__rom_region_start` to `__rom_region_size`)
as read-only and executable (PMP_R | PMP_X | PMP_L).

This change wraps the setup of this default ROM PMP entry with
`#ifndef CONFIG_CUSTOM_PMP_ENTRY`.

If `CONFIG_CUSTOM_PMP_ENTRY` is set, this default ROM entry configuration
is skipped. This allows projects, SoCs, or board configurations to provide a
completely custom PMP layout starting from the first entry from the
device tree based on `zephyr,memattr`, which might be necessary for
specialized memory protection schemes or to override the default ROM
region attributes.

Signed-off-by: Firas Sammoura <[email protected]>

Author: fsammoura1980

arch/riscv/core/pmp.c

@@ -373,12 +373,16 @@ void z_riscv_pmp_init(void)
 	}
 #endif
 
+#ifndef CONFIG_CUSTOM_PMP_ENTRY
+
 	/* The read-only area is always there for every mode */
 	set_pmp_entry(&index, PMP_R | PMP_X | PMP_L,
 		      (uintptr_t)__rom_region_start,
 		      (size_t)__rom_region_size,
 		      pmp_addr, pmp_cfg, ARRAY_SIZE(pmp_addr));
 
+#endif
+
 #ifdef CONFIG_NULL_POINTER_EXCEPTION_DETECTION_PMP
 	/*
 	 * Use a PMP slot to make region (starting at address 0x0) inaccessible