ti: AM43xx: board: Detect AM43xx HS EVM

Adds code to detect AM43xx HS EVMS - the string in the
I2C EEPROM for HS EVMs differs from GP EVMs. Adds code to
for evm detection, regardless of whether the evm is for
GP or HS parts, and updates board init to use that.

Modifies findfdt command to pick up am437x-gp-evm.dtb for
the HS EVMs also, as the boards are similar except for
some security specific changes around power supply and
enclosure protection.

Signed-off-by: Madan Srinivas <madans@ti.com>
Signed-off-by: Daniel Allred <d-allred@ti.com>
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>

Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>