HV: refine acpi rsdp initialize interface

In previous code, the rsdp initialization is done in get_rsdp() api implicitly.
The function is called multiple times in following acpi table parsing functions
and the condition (rsdp == NULL) need to be added in each parsing function.
This is not needed since the panic would occur if rsdp is NULL when do acpi
initialization.

Tracked-On: #5626

Signed-off-by: Victor Sun <victor.sun@intel.com>
Reviewed-by: Jason Chen CJ <jason.cj.chen@intel.com>

hypervisor/boot/include/acpi.h

@@ -236,6 +236,7 @@ struct acpi_table_tpm2 {
 	uint32_t start_method;
 } __packed;
 
+void init_acpi(void);
 void *get_acpi_tbl(const char *signature);
 
 struct ioapic_info;