hv: Other preparation for trampoline code relocation

- For UEFI boot, allocate memory for trampoline code in ACRN EFI,
  and pass the pointer to HV through efi_ctx
- Correct LOW_RAM_SIZE and LOW_RAM_START in Kconfig and bsp_cfg.h
- use trampline_start16_paddr instead of the hardcoded
  CONFIG_LOW_RAM_START for initial guest GDT and page tables

Signed-off-by: Zheng, Gen <gen.zheng@intel.com>
Signed-off-by: Jason Chen CJ <jason.cj.chen@intel.com>
Signed-off-by: Zide Chen <zide.chen@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>
Acked-by: Xu, Anthony <anthony.xu@intel.com>

hypervisor/include/arch/x86/cpu.h

@@ -169,6 +169,7 @@ extern uint8_t                trampline_pdpt_addr[];
 extern uint8_t                trampline_gdt_ptr[];
 extern uint8_t                trampline_start64_fixup[];
 
+extern uint64_t trampline_start16_paddr;
 extern int ibrs_type;
 
 /*

hypervisor/include/common/acrn_efi.h

@@ -15,6 +15,7 @@ typedef struct {
 struct efi_ctx {
 	uint64_t rip;
 	void *rsdp;
+	void *ap_trampline_buf;
 	dt_addr_t  gdt;
 	dt_addr_t  idt;
 	uint16_t   tr_sel;
@@ -50,5 +51,6 @@ struct efi_ctx {
 }__attribute__((packed));
 
 void *get_rsdp_from_uefi(void);
+void *get_ap_trampline_buf(void);
 
 #endif /* UEFI_H*/