board: htc: endeavoru: add an early SPL RCM hook

Unlike all other supported Tegra devices and most known Tegra devices, the
HTC One X has no hardware way to enter APX/RCM mode, which may lead to a
dangerous situation when, if BCT is set correctly and the bootloader is
faulty, the device will hang in a permanent brick state. Exiting from this
state can be done only by disassembling the device and shortening the
testpad to the ground.

To prevent this or to minimize the probability of such an accident, it was
proposed to add the RCM rebooting hook as early into SPL as possible since
SPL is much more robust and has minimal changes that can break bootflow.

gpio_early_init_uart() function was chosen as it is the earliest function
exposed for setup by the device. Hook performs a check for volume up button
state and triggers RCM if it is pressed.

Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>