Fix SDM detection for P4

[bugadani]

On the P4, the old magic address of 0x40001000 is actually external memory, and trying to read that just gives a timeout. This causes espflash to mis-detect secure download mode.

With this PR, espflash board-info --no-stub now responds with

Chip type:         esp32p4 (revision v3.0)
Crystal frequency: 40 MHz
Flash size:        16MB
Features:          High-Performance MCU
MAC address:       30:ed:a0:ec:f1:01

Security Information:
=====================
Flags: 0x00000000 (0)
Key Purposes: [0, 0, 0, 0, 0, 0, 19]
Chip ID: 18
API Version: 5
Secure Boot: Disabled
Flash Encryption: Disabled
SPI Boot Crypt Count (SPI_BOOT_CRYPT_CNT): 0x0

[SergioGasquez]

LGTM! Thanks